io.emit vs socket.emit
io.emit lets the server send an event to all the connected clients at once. This is useful when you need to send some data to everyone. This is something we want everyone to be able to see.
socket.emit sends an event to a single socket. This is great when you need to send an event to a single user. Example: This is why we use socket.emit when sending out a greeting message. It only needs to go to that one user.