Question

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

Answer 1

Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult.
For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.

Answer 2

Variations are beneficial for the survival of the species. Populations of organisms fill well-defined places, or niches, in the ecosystem, using their ability to reproduce. However, niches can change because of reasons beyond the control of the organisms and could even wipe out the entire population. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive. 

For example - If there were a population of bacteria living in temperate waters and suppose the water temperature were to be increased by global warming. Most of these bacteria would die, but the few heat resistant variants would survive and grow further. Variation is thus useful for the survival of species over time.