How can variation in offspring be beneficial to survival of a population?

If every organism in a population had the same genes, then they would all be affected in exactly the same way by any outside factors from the environment.

This means that if some factor resulted in those genes being lethal or not suited to survival, the entire population would suffer in exactly the same way: they would all die and the species becomes extinct. The idiom "all your eggs in one basket" comes to mind when describing such a situation.

Variation allows for some organisms to survive different factors. If some genes were lethal or not suited for survival, then only the affected members die and the majority (or at least enough to ensure continuation) of the population survive.