Question

Why is variation beneficial to the species but not necessarily for the individuals?

Answer 1

Due to the presence of variations, few individuals in populations may survive under extreme conditions like change in temperature, varying water level, etc and if environment drastically altered, population except a few individuals with variations can be wiped out. this in belton variation becomes the basis of evolution and overtime such variations are useful for the survival of species.

Answer 2

Due to creation of variations, a species can adjust to the changing environment around it. And this promotes the survival of the species in the changing environment. For example, the accumulation of 'heat resistant' trait or variation in some bacteria will ensure its survival even when the temperature in its environment  rises too much due to a heat wave (just a reason for the example). On the other hand bacteria which do not have this trait or variation will not be able to withstand the heat and die out.

The altered niche(changed environment) will only be a ground for survival for those individuals with variation. So the species will continue to exist, but the individuals without the variation will not benefit from the altered niche.

When you think about it, its like the maze runner series, where Thomas and his friends have the variation to survive the virus-type-thing.

So yeah, variation is highly beneficial to ensure the existence of a species, but not for those individuals who don't have the variation.

Hope it helps,

Stay cool!