In very basic terms, you fly fast because passengers generally don't want to spend four hours in the air if they can get there in three. You fly fast because if you can get there in three hours rather than four then you can get more aircraft cycles in during the same time period and aircraft only make money whilst they're in the air. Finally to some degree, you fly fast because jet engines are more fuel efficient when working hard, so your aircraft will burn comparitively less fuel flying fast than it will flying slow.
You fly high because (for a variety of technical reasons which I won't go into) basically the higher you fly, the faster you fly. See first paragraph.
This isn't chapter and verse, but I'm guessing from the question you're not qualified - so hopefully it's enough to give you the broad idea.