Makes the sex cells necessary for sexual reproduction

"Sex Cells" or "haploids" are needed in order to add variation in DNA. If there was no "haploids" (which are just cells with 23 Chromosomes), then the female would not need the male for reproduction, basically meaning birth of an identical twin to the mother. The two haploids, the female's ova (Egg) and male's sperm meet, creating a zygote, or a fertilized egg.

This makes sure there is "immediate variation", and it isn't just ten generations of the same "Bob"!