Genetic variation is important because it results in 1) new alleles from mutations and 2) natural selection, one of the most important driving forces in evolution. Genetic variation keeps a population healthy.
For example, cheetahs are a species that does not have much genetic variation. Therefore if some kind of disease began to harm the cheetahs, they are at heavy risk of going extinct because none of them would have a mutation that might make them immune to it.
Hope that helped you.