To answer the question about why we sleep, we need to understand that sleep (or dormancy) is a nearly universal life phenomenon at least as far as earth life forms go. Organisms go dormant periodically. If one examines the periodicity of dormancy a few interesting observations can be made. Dormancy happens usually as a function of available energy. When the sunlight goes away, for example, many organisms go dormant. If the sunlight also reduces heat energy for prolonged periods of time, the dormancy period can extend into a state we call hibernation.
Organisms that cannot sustain themselves throughout a period of reduced energy availability are doomed to fail to reproduce. However, there is a challenge to the organism to maintain essential life functions during periods of lowered energy availability. A bit of thought will allow us to conclude that from the beginning of life on earth, this must have been one of life's first challenges, i.e. to survive for the period of time when sunlight energy was not readily available overhead. Once the first life forms mastered this task by going into a period of dormancy until the sun rose again, the pattern for sleep was established in almost all future iterations of life on this planet, albeit in a wonderful array of permutations and combinations.
So the answer to why we sleep really boils down to the answer, "because the earth turns."