Socket.io is useful when you have legacy web browsers that may not do websockets. Beyond that, it does not bring much over regular websockets.
Not on the web it isn't.
I agree that socket.io offers little over WebSocket except for backwards-compatibility, but the OP's question was more about why you'd want to use WebSocket.
Socket.io is useful when you have legacy web browsers that may not do websockets. Beyond that, it does not bring much over regular websockets.