In India they want you to honk your horn

Where I'm from, honking is considered offensive. Here it's considered a courtesy, considerate, even.

My guess is the reason is that the traffic is so crazy you have to focus your full attention forward. So if someone's coming from behind, they'd better let you know, or you might go ahead and change lanes right as they're overtaking you.

That's why you see most trucks have the words "Horn Please" painted on the back.