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.
It's been a while since I've been to Vietnam but most of the traffic is composed of motorcycle and not cars, so honking is indeed a signal of presence that's needed compared to just having the noise of a single car that's behind you.
(crossing the street is also kind of surreal as it's more like going through a school of fish; the trick is to walk at a steady pace to maximize your position predictability)
When I got my driving license too many years ago in Italy, they taught me that a brief honk when taking over in interurban roads is actually mandatory (but nobody did it). I don't know if that rule stayed the same as they harmonized more and more the road rules to the European ones.
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.