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

It allows the individuals in a population to survive and have offspring with the desired trait needed to live while those without it will die out.

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