As someone who has grown up (and learnt to drive) in England - I had a moment of sudden realisation in India. For me, the horn is a warning sign - "get out of my way" or "watch out!" - only to be used in emergencies. In India, the horn means "I'm over here", to be used whenever you're near another vehicle. Which fits with the statements on the back of the lorries perfectly.
Here in Massachusetts, it seems the horn has only three functions. The first, a quick "beep beep" signals that you recognize another driver or pedestrian and are saying hello. The second, a one to three second beep means that the traffic light has switched to green and car in front of you has not begun moving within 20ms. And the third, a five to sixty second "beeeeeeeeep" means that you consider the other driver to have made a mistake and needs to understand their very, very low place in the social hierarchy. This third one is the most used.