What is ERC677? When to use it?
Topics are indexed parameters to an event. EVM uses low-level primitives called logs to map them to high-level solidity construct called Event.
ERC677
Allow tokens to be transferred to contracts and have the contract trigger logic for how to respond to receiving the tokens within a single transaction.
To do an api call or vrf request requires a function call on a smart contract (ie on the the oracle contract or operator contract or vrf coordinator contract etc). To transfer a token also requires a function call on the token contract. So if you say wanted to do a vrf request and send the payment in link, usually that would be 2 seperate function calls on 2 seperate contracts. With erc677 you can basically do both in a single transaction, thanks to the magic of the onTokenTransfer and transferAndCall functions.