In Asia it's mainly used to signal your presence, like when you're overtaking someone. Just a little tap of the horn. Just the fact that you hear it say in the right corner behind you will make you not swerve in that direction. It's almost subconscious and really does improve safety imho. You can't possibly visually scan for all vehicles around you.

The result of course is that there's a non stop cacophony, in places like Hanoi it REALLY gets to you after a while.

Here in EU if someone honks at you it's considered rude and will make me really react with wtf is your problem. Out in Asia it's completely normal.