Answer:
To develop more favourable traits in offspring for survival.
I'm no bio expert but from my understanding, it's better to reproduce sexually to broaden the gene pool and allow for the offspring to develop better traits for survival.
for example if the parent is susceptible to a certain disease then asexual will cause the offspring to also be susceptible. however, through sexual reproduction with a parent who is immune, the offspring could be immune as well.
Hope that helps