The main point is selection at the level of individual sperm. Sperm are very cheap, but carrying an infant to term is very expensive. It is an absolute requirement to avoid carrying an infant with genetic disorders, to whatever extent possible. So the male creates a million sperm, and the female puts them through an obstacle course. A sperm with genetic defects probably won't even survive in the harsh environment, let alone win the race. So you're guaranteed that the winner of the Fallopian race is at least basically healthy, with basic functioning metabolic machinery and so on.
