It is extremely important to have genetic variation with a species or population for the following reason; the smaller amount of variation in a pool of genetics the more likely the offspring will have a mutation or die. Therefore you need to have genetic variation in the genetic pool because without variation reproduction is limited and all of them could die.
It is extremely important to have a specific size of population. For example the smaller the population is the more likely there will be mistakes due to the fact that the genetic pool is limited. Therefore it is better to have a bigger population with a larger amount of animals that provide a large amount of variation. If you have a larger population the amount of genetic mistakes that could be possibly made chance of occurring decreases.