Superficially, evolutionary theory suggests that asexual reproduction should be the norm. Given the paucity of obligate asexual organisms, there must be an advantage to sexual reproduction. The benefits of sexuality Sex is not just for selling cars or occupying time during power cuts. It must have an evolutionary advantage to explain its ubiquity. There are dozens of theories circulating, some of them quite absurd.
Mutations occur all the time.
In asexual species, once a mutation occurs it will be passed on to all future offspring. Since the vast majority of mutations are harmful, this means that asexual organisms will tend to accumulate mutations over time. Sexual organisms, however, mix their genes. So even if a parent contains a mutant gene, it has only one chance in two of passing that gene onto its offspring. And since the offspring will get a non-mutant copy of the same gene from the other parent, that means that in all likelihood the mutation will not destroy the organism's chance of reproducing, even if the mutation is harmful.