Why do we need the MP variable MPHOST? The host is always no. 0, isn't it?

I haven't used any multiplayer instructions yet (mainly because I lack another 3DS with SmileBASIC), but if it's true that the host's terminal ID is always 0, then I guess its use could be for code readability purposes.

I don't think that it's always 0. Unless you have confirmed this by testing multiplayer functions, it's likely not always 0. MPHOST is the terminal of the host. If you don't host the connection (I believe there is a different command for hosting and joining, correct me if I'm wrong), then MPHOST will not be 0. The individual system's terminal will always be 0, but not MPHOST.

I know how to set multiplier, but how do you code a game with it?

Ask that in my discussion.