why is the variation beneficial to the species but not for the individual?

Variation is useful for the survival of the species over time. If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population would be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive. These resistant variants would survive and grow further. So over time, these variations will prove useful for the species.