Simplistic answer:
Jets don't like being throttled back, i.e. they are most efficient at approx 100% design rpm.
At low level 100% rpm produces way too much thrust, fine for take off, too much to crz without pulling wings off. (Higher mass flow of air means higher mass flow of fuel)
As alt increased, amount of thrust goes down until you don't have to throttle back too much to crz comfortably so engine more efficient. (Mainly due to optimised gas flows internally).
So - higher means engine operates nearer optimal regime.
Of course there are other advantages - increased TAS being the obvious one. The optimum level is a compromise between what the airframe likes and what the engine likes.
If you want a jet a/c optimised for low level build it so it requires four engines to take off, then shut two down once you're airborne. (I believe that's what the RAF do with Nimrods if loitering low level for long periods)