Why is it an advantage to be able to reproduce sexually and asexually?

It's an advantage to both produce sexually and a sexually because by producing sexually, there will be more genetic diversity in the offspring, thus population, and by producing asexually, if there is no potential mates, breeding by oneself can also carry on another generation. 
Sexually: Advantage: genetic diversity. Disadvantage: No mate=No offspring.
Asexually: Advantage: No need for the search for a mate/partner. Disadvantage: Basically clones of oneself=No genetic diversity>leads to high susceptibility to being wiped put by deadly diseases.

Sexually- In sexual reproduction, two parents contribute genetic information to produce unique offspring. and sometimes it would be hard for an organism to find a mate

Asexually- In Asexual reproduction, a organism wont have to find a mate in order to reproduce, so it can create offspring by itself, and enable the continuation of its' species you know without having the struggle of finding a mate.

Hopefully this helped ^.^ Mark if possible~