On solidity online compiler along with create contract button/div there is another placeholder At Address what does it do?
If you point Remix to any one of the ethereum networks (ropsten for example), you can use remix to interact with any contract that is deployed on the network already.
This is a very handy way of debugging your deployed contract without writing clientside code as remix generates the interactive UI.
Another great use case is to interact with verified contracts on etherscan.io
take this erc20 token for example: https://etherscan.io/address/0xAef38fBFBF932D1AeF3B808Bc8fBd8Cd8E1f8BC5#code
remix is super powerful! for one-offs though, I tend to use ethfiddle.com as has built-in sharing