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

If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there may be some chance of their survival.

E.g.: If the temperature of water increases by global warming, most bacteria would die. But a population of heat resistant bacteria could survive here. These 

few variants would be resistant to the heat and

would survive and grow further. Variation is thus useful for the survival of species over time.