In a complex system such as life, any little genotypic change which gives an organism a distinct advantage will be "exploited". It is therefore a good strategy to have as much diversity in your progenies as possible. It is fortunate that replication anomalies like mutation and cross-over can occur. Otherwise, what we have is a species of organism that cannot improve upon itself; any change in its environment may be fatal.Life finds a way.
Sexual reproduction multiplies the potential of these replication anomalies to create diversity since it involves the mixing of genes from two individuals. Asexual reproduction on the other hand involves the genes of only one individual. The advantage of the former is obvious: the more diverse the progenies are, the more likely a better solution to the problem space of life will be found.
It is conceivable of course for life not to have stumbled upon sexual reproduction at all and thus failed to reap its benefits. It is also conceivable for life to have evolved a manner of sexual reproduction wherein 3 partners are involved instead of just 2, thus assuring more diversity. This line of thought I leave to science fiction.