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
just copy the contract code here:
then paste it into remix: now you can interact with a contract on mainnet!
remix is super powerful! for one-offs though, I tend to use ethfiddle.com as has built-in sharing
You can enter the address of an already deployed contract to load it and be able to call its functions.
Specially useful when you have a contract that deploys other contracts and you eant to interact with those contracts.