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

Variation is necessary for the survival of species as variation makes species more adapted to survive with the changing conditions. At the species level these variations can make difference at each generation but at individual level an organism is not evolving but is ageing. If an individual naturally accumulates variation in his gene composition through mutation or any other source could be harmful. Thus variation at individual level is not so useful as a person does not evolve by these variations.