In sexual reproduction, two individuals having different variations combine their $DNA$ to give rise to a new individual. Therefore, sexual reproduction allows more variations, whereas in asexual reproduction, chance variations can only occur when the copying of $DNA$ is not accurate.

Additionally, asexual reproduction allows very less variations because if there are more variations, then the resultant $DNA$ will not be able to survive inside the inherited cellular apparatus.

However, in sexual reproduction, more variations are allowed and the resultant  $DNA$  is also able to survive, thus making the variations viable.

Variation and Evolution : Variants help the species to survive in all the conditions. Environmental conditions such as heat, light, pests, and food availability can change suddenly at only one place. At that time, only those variants resistant to these conditions would be able to survive. This will slowly lead to the evolution of a better adapted species. Thus, variation helps in the evolution of sexually reproducing organisms.