Question

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

Answer 1

Variation is an important aspect of evolution that helps the species of organisms to survive in harsh conditions and prevents them from going extinct. The variation in organisms is selected by nature only and the process is utterly necessary for the survival of the species.

Complete answer:
The statement mentioned in the question that variation is beneficial to the species but not necessarily for the individual refers to the fact that variation helps the species to survive but may not help the individual. Rather, it may cause harm to the individual or even cause death.
If we consider that a change occurs in a species and some variations develop. Those variants may be able to cope with the change and have a chance of survival through the harsh condition. But the other members of the species will not survive. But in the same situation, if the variation occurs at an individual's level the individual may not cope up in that unsuitable environment and dies. So, we can say that variation is helpful at a species-level for evolution but at an individual's level, it is not necessarily beneficial.
So there remains the equal possibility that if a drastic change occurs in the species it will either evolve and show a variation or will be eliminated from the ecosystem. The impact will be the same on every individual but a variation can save the species from going extinct.

Note: Variation and natural selection are interrelated in a way that variation happens only in naturally selected organisms and both together form the basis of evolution. An example of such evolution is the appearance of heat resistance in bacteria among normal bacteria. The group of heat-resistant bacteria will survive even if the temperature of water rises due to global warming and carry on the generations of the species whereas the normal bacteria will die.