Many feel that sidechains will be necessary moving forward. There is already a question about scalability with blockchain networks. Sidechains allow for the integration of more nodes, allowing for transactions to be moved off the main network.
Consider the main blockchain to be pristine real estate. This is the high rent district. Since blockchains are ledgers, data is stored in a decentralized manner. This provides immutability and censorship resistance to whatever is posted.
Not everything needs to be housed in the most expensive part of town. Users will have the option of where to post the data. Sidechains can be thought of as industrial areas where the real estate is far less expensive. While there are not as many features available, such as the censorship resistance, the cost is more palatable.
As the blockchain world expands, interoperability is vital. The early chains such as Litecoin and Ethereum utilize different languages. For this reason, the ability to bridge between different networks and access the features is going to be vital.
One requirement is for sidechains to remove counterparty risk. This is done by having a two-way peg on assets showing up on both chains. This allows for seamless transfer of value over the different network, eliminating costs that would typically be an opportunity for arbitrage.
Also, because of the peg, nobody can stop the transfer from happening.
Transfer is a conceptual term since the original assets never leave the original chain. There is an equivalent produced on the other chain. This is often called a wrapped token which is an replica of what is on the main chain, with the value transferred to the new chain.
This allows for it to be "transferred" back and forth between both networks.