Side chains have two main advantages. Their first advantage they have is that they are permanent. You do not have to create a new sidechain every time you need to use one. Once a side chain is built, it is maintained and can be used by anyone doing a specified task off the main chain. The other advantage of sidechains is that they allow interaction between different cryptocurrencies. Developers get the opportunity to test software upgrades as well as beta coin releases before they are released on the main chain.
It doesn’t matter if you’re moving $1bn or 0.01c across the Bitcoin network, you get the same security guarantees. And you pay for this in fees and time. What if you were prepared to trade safety for speed? Today, your only real option is to send the coins to a centralized wallet provider, whom you must trust not to lose or steal your coins. You can then do all the transactions you like on their books, with their other customers and you never need touch the Bitcoin blockchain. But now you lose all the benefits of a decentralized value-transfer network.
Write permissions are kept centralized to one organization. Read permissions may be public or restricted to an arbitrary extent. Example applications include database management, auditing, etc. which are internal to a single company, and so public readability may in many cases not be necessary at all. In other cases public audit ability is desired. Private blockchains are a way of taking advantage of blockchain technology by setting up groups and participants who can verify transactions internally. This puts you at the risk of security breaches just like in a centralized system, as opposed to public blockchain secured by game theoretic incentive mechanisms. However, private blockchains have their use case, especially when it comes to scalability and state compliance of data privacy rules and other regulatory issues. They have certain security advantages, and other security disadvantages (as stated before).
The cheapest and most simple option is doing calculations on your local network (off-chain) and integrating with main blockchain by sending the results. It has flaws; you cannot live full advantage of blockchain as we do in bitcoin, because you will still have existing constraints of your current system. Despite all this, it is still a valid option; perhaps you won't need all the features of blockchain technology. Perhaps it is just enough to use blockchain only for your pain points. Factom can be considered under that kind of option. They used bitcoin wisely in their design. They hold the actual mass data in their network and utilize stability of bitcoin in their solution. This project is so successful that at coindesk magazine, it is saying that Factom can be used for the land titles in Honduras. http://www.coindesk.com/debate-f...
The immense promise and accelerated development of permissioned blockchain technology, combined with intense business interest from a wide range of industries, is acting as a perfect stimulant for more and more enterprises to start rolling out blockchain networks into production. I envision these permissioned networks will soon directly or indirectly influence every facet of human enterprise.