“Given all of this, it may seem like private blockchains are unquestionably a better choice for institutions. However, even in an institutional context, public blockchains still have a lot of value, and, in fact, this value lies to a substantial degree in the philosophical virtues that advocates of public blockchains have been promoting all along, among the chief of which are freedom, neutrality and openness.” 
The main point of a side-chain is to allow cryptocurrency networks to scale and interact with one-another. For example alt-coins and Bitcoin run on separate chains, however side chains allow for these separate currencies to be transferred through these two-way 'portal's or interfaces via a fixed conversion amount. Added benefits of side-chains are different asset classes like,stocks, bonds etc being integrated through a converted price onto the main chain, along with additional functionality like smart contracts,unique D-Apps, micro-payments and security updates that can be later incorporated into the primary network from these side-chains.
By contrast, the Bitcoin blockchain is not Turing complete since it has little to no ability for data manipulation. It has no ability for a user to deploy if else or goto statements. This is a bit of a simplification but anytime you hear someone say something is “Turing complete” you can do a quick check to see if there is functionality for data changes, memory changes and if/else statements. If there is, that’s usually what they mean.

Transactions are cheaper, since they only need to be verified by a few nodes that can be trusted to have very high processing power, and do not need to be verified by ten thousand laptops. This is a hugely important concern right now, as public blockchains tend to have transaction fees exceeding $0.01 per tx, but it is important to note that it may change in the long term with scalable blockchain technology that promises to bring public-blockchain costs down to within one or two orders of magnitude of an optimally efficient private blockchain system

Open blockchains are more user-friendly than some traditional ownership records, which, while open to the public, still require physical access to view. Because all early blockchains were permissionless, controversy has arisen over the blockchain definition. An issue in this ongoing debate is whether a private system with verifiers tasked and authorized (permissioned) by a central authority should be considered a blockchain.[36][37][38][39][40] Proponents of permissioned or private chains argue that the term "blockchain" may be applied to any data structure that batches data into time-stamped blocks. These blockchains serve as a distributed version of multiversion concurrency control (MVCC) in databases.[41] Just as MVCC prevents two transactions from concurrently modifying a single object in a database, blockchains prevent two transactions from spending the same single output in a blockchain.[42]:30–31 Opponents say that permissioned systems resemble traditional corporate databases, not supporting decentralized data verification, and that such systems are not hardened against operator tampering and revision.[36][38] Nikolai Hampton of Computerworld said that "many in-house blockchain solutions will be nothing more than cumbersome databases," and "without a clear security model, proprietary blockchains should be eyed with suspicion."[9][43]

For example, Banks A and B often settle thousands of transactions per day. It would be extremely expensive for all of those transactions to be committed to the main blockchain, so A and B set up a side-chain. At the end of each day, at most one transaction is committed to the main blockchain (the only possible outcomes are A and B's balances remain the same, or one of their balances decreases and the other's increases).

– we provide no uniqueness of names, unlike the domain registrars, social networks, namecoin, onename.io, etc. There is no uniqueness of names in real life either. Instead the identity is just a hash of a [json] object that contains a public key. Identity object can not be modified directly, but a new version of it can be created, pointing to a previous version. The owner of the identity object can optionally connect it with the real life credentials, e.g. the social account, internet domain, email, etc. by proving the proof of ownership of that account the way onetime.io does it, the way Google Analytics does it, etc. This allows a spectrum of identities from fully anonymous to fully disclosed and verified. This also allows a person to have multiple identities, for work, for social, for gaming, for interest-specific forums. To simulate OAUTH2, a new site-specific identity can be created and signed with person’s other identity.

Necessity is the answer to that question, well for the short term anyway. Currently public & private blockchains still have a lot of challenging technological problems that need to be sorted out, with privacy and scalability being foremost. Gallactic’s blockchain can certainly help with scalability due to its multi-chain architecture that allows for massive scaling to rival and in most cases surpass other blockchains in the market with transactions at 300 per second on mainchain with the ability to scale up to hundreds of thousands per second when the multi-chain model is configured for speed.
A consortium blockchain is part public, part private. This split works at the level of the consensus process: on a consortium chain, a pre-selected group of nodes control the consensus process, but other nodes may be allowed to participate in creating new transactions and/or reviewing it. The specific configuration of each consortium chain (i.e., which nodes have the power to authorize transactions via the consensus process, which can review the history of the chain, which can create new transactions, and more) is the decision of each individual consortium.
Frankly, secure implementation of Bitcoin is already a pain in the ass .. adding more complexity just seems like the wrong move at this point. It’s already trying to be a currency, a networking protocol and a client in the same codebase. Adding turing complete (or not) scripts with arbitrary outcomes, multiple versions of the official client cooperating, multiple clients, and now multiple blockchains is basically the nail in the coffin in terms of widespread implementation.

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Blockstream recently released a whitepaper on “strong federations,” which is essentially their vision of a federated two-way peg system. Liquid is a sidechain created by Blockstream that uses the strong federations model. The sidechain is used to transfer bitcoins between centralized bitcoin institutions, such as exchanges, at a faster pace than the public Bitcoin blockchain.

“Blockchain offers a possible solution to these challenges with its decentralized ledger that can store a history of transactions across a shared database,” Cohen said in the report. “By making the record accessible and verifiable from anywhere in the world, blockchain can enable the authentication of goods and eradicate the criminal element of counterfeit goods in the retail supply chain. By pairing hardware chips with blockchain technology, a product can take on a digital history, going as far back as the raw materials that were used to make the product. This allows retailers and consumers to verify their purchased products are genuine.”
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thank you for the clear explanation of this. so in essence, by locking bitcoins to a particular address we’ve created an asset (collateral). then on the other sidechain (marketplace) we get issued shares against the asset, which we can sell. anyone holding a share can then redeem it against the asset. I think that’s an analogy that finance types would get
Blockchain was invented by Satoshi Nakamoto in 2008 to serve as the public transaction ledger of the cryptocurrency bitcoin.[1] The invention of the blockchain for bitcoin made it the first digital currency to solve the double-spending problem without the need of a trusted authority or central server. The bitcoin design has inspired other applications,[1][3] and blockchains which are readable by the public are widely used by cryptocurrencies. Private blockchains have been proposed for business use. Some marketing of blockchains has been called "snake oil".[9]
Jump up ^ Iansiti, Marco; Lakhani, Karim R. (January 2017). "The Truth About Blockchain". Harvard Business Review. Harvard University. Archived from the original on 18 January 2017. Retrieved 17 January 2017. The technology at the heart of bitcoin and other virtual currencies, blockchain is an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way.
The first work on a cryptographically secured chain of blocks was described in 1991 by Stuart Haber and W. Scott Stornetta.[10][6] They wanted to implement a system where documents' timestamps could not be tampered with or backdated. In 1992, Bayer, Haber and Stornetta incorporated Merkle trees to the design, which improved its efficiency by allowing several documents to be collected into one block.[6][11]

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thank you for the clear explanation of this. so in essence, by locking bitcoins to a particular address we’ve created an asset (collateral). then on the other sidechain (marketplace) we get issued shares against the asset, which we can sell. anyone holding a share can then redeem it against the asset. I think that’s an analogy that finance types would get

Instead, what if the game was played in its own “channel”? Each time a player made a move, the state of the game is signed by each player. After an epic battle where the Protoss player takes out the remaining Zerg forces and forces a gg, the final state of the game (Protoss wins) is sent to a smart contract on the main chain. This neutral smart contract, known as a Judge, waits a while to see if the Zerg player disputes the outcome. If the Zerg player doesn’t, the Protoss player is paid the 1 ETH. </injects>