>>100602635Different sexes allow the mixing of genes in different combinations during the reproductive process, allowing offspring to possibly be better at living against whatever adverse effects are acting on the population.
Compare to asexual reproduction, where offspring are by and large identical to their parent organisms, and as such will face the same biological or environmental hardships the same way.
With asexual reproduction the only chance for genetic variation is by random mutation gene errors, while sexual reproduction has genetic variation built into the productive process.