Bonus... why?

Can someone please explain to me why a Bonus must be paid to anyone at all?

When a person applies for a job, does that person at the interview time not promise the potential employer to do his utmost to promote the well-being of the organisation? Or does the person say: "Sir, I am only making available some of my knowledge / experience / know-how / skills to the organisation?".

Yes, a bonus must be paid if a person has delivered to an organisation some benefit that does NOT lie within his ability to deliver or to put it plainly, does not fit into his job description.

I certainly cannot think of one instance why a top executive in an organisation should be paid a bonus either. A top executive's raison d'être in an organisation is to increase the value of the organisation. A top executive's outlook is to-day, to-morrow and naturally also the day after to-morrow!