Well, it does the repairing and all that while we're asleep because the body is busy using up energy on other things while we're awake. We'd do the same thing if we stayed awake all the time, just slower and less efficiently.

Sleep experiments are somewhat inconclusive as well; one experiment had a dude stay up for 5 days, and he was pretty messed up. He was having delusions, and all of his cognitive and motor functions were impaired. Yet, another fellow stayed up for weeks with no definitive side effects; beat the doctor monitoring him in 100 consecutive games of ping pong. Went to sleep, was perfectly fine the next day.

Weird how the body works.