In Pune people use horns to:
1) Inform someone that they have arrived to pick them up
2) Scare a senior citizen off a zebra from crossing the road
3) Get some law abiding driver ahead of them to move on in a red light
Lets not forget that the horn is meant to warn people or other drivers getting in way of a moving vehicle or breaking some traffic rules.
Indiscriminate use is not only rude but also a public nuisance. Often people who arrive to pick up somebody prefer to honk. The frequency and intensity increases if there is no response from the intended person as he or she often remains blissfully unaware. The entire neighbourhood gets thoroughly disturbed. Remember there could be old or sick people and students preparing for exams. Some basic courtesies have to be followed. Go up yourself or use a cell phone.
No one wants to make a permanent halt at any signal. Do give a moment before the vehicle ahead of you moves on instead of blasting away on the horn. There could be an unexpected pedestrian or some genuine reason why his progress is delayed by a fraction.
Do show respect to senior citizens who want to cross roads. Many have physical limitations (failing eyesight, joint problems, hearing issues etc) which reduces their walking speed. All of us will reach this stage at some time.
If you have to use auditory systems in your vehicle to warn people ahead or express your anger, do show some discretion.
I am not really sure of the origin or the sign 'Horn Ok Please' often seen on the rear of trucks.
It is definitely Not OK to blow the horn without reason and irresponsibly.