Why does genetic variation increase the chance to some indivuals in a population will survive

It enables the population to adapt more efuceantly to survive

Explanation:

any undesired trait will be eliminated by survival of the fittest and the wanted traits will be passed on

one example could be that a certain population of moths are white, becouse of this they are easily spotted and more likely to be eaten. Becouse of genetic variation a brown mutation of the moths is created. The brown one lives and passes on the trait, eventually the population will mostly be brown instead of white