as far as i remember, mqtt is for at the bare minimum sending/recieving messages to/from different snap clients who are "subscribed" to recieve those messages. this can be used for multiplayer games (although the speeds arent great, as far as i remember), and just anything else that involves sending/recieving information to/from another snap client.
heres an example of a multiplayer tic-tac-toe game i made a while back using the mqtt blocks