The safety part does have something to do with it. But more often the horn is used as a stress ball, as if pressing it will somehow get traffic moving faster.
There's also this category of bikers who want to speed on empty roads looking all "heroic" who honk away without a care in the world.
I am from Bangalore, and in 2012, I tried an experiment in an auto-rickshaw (tuk-tuk). I offered to pay the driver double the meter price if he didn't honk even once during an upcoming 1 hour-ish ride.
The driver thought I was nuts to offer him this obviously easy money, and accepted it. I told him up front that the first honk, and the extra money was off the table. He said - ok - and we started the ride. The first few kilometers were pleasant, and the driver was cool with it - but as we entered the business district with more traffic, he became more and more frustrated and started to hum and sing to take it out a bit. Then, he kind of started "barking" (for the lack of a better word) at other drivers who irritated him. He then resorted to outright shouting at the drivers. Eventually, something snapped and he honked once. I told him the deal was off, and the rest of my trip was a veritable honk-hell.
I have always believed the stress ball theory after this one-off experiment.
