Why the fuck people honk?

It pisses the crap out of me, when someone honks at a stop light. If it is green and the guy already started moving why the fuck people still honk?

Is it to tell him/her that they did a great job taking off on green?

I understand if the first car is stopped and it has been green for a bit already, but yesterday in 2 occasion some fucking idiot honked to someone that did nothing wrong.

When someone honks at me for now reason, is guaranteed a slow down, and at least a middle finger wave.