Mutations. You're going to get way more variation over time with sexual transmission of variation than you would likely get from having to rely on mutations of self-replicating organizisms. Plus, a successful mutation will spread far more rapidly where there is sex than where there is none. If there is a successful mutation of one self-replicating "female", that mutation can be passed on only to that female's offspring. But if you have a successful mutation in a male, that success can be passed on to a much larger number of females.
You're pre-supposing the existence of males. But how you get males and females that can mate and produce fertile male/female offspring? Where does the mating behavior come from?