Genetic variation is the process for genetic change to occur. Without variants within genes, no different genotype can evolve; if the genetic material is forever "stagnant," later generations would be the same as prior ones and there would be no ability for species to adapt.
Evidence suggests that all the dinosaurs did not die off 65 million years ago; some of them were able to evolve into birds. Dinosaurs that had a slightly different genotype from their peers might have been able to weather the environmental changes and evolve into a life form more suited to existing conditions.
However, variation is a two way street -- some genotypes may make an organism less suited to its environment, or cause other systemic problems. Individuals with the variant genes that confer resistance to sleeping sickness could, at the same time, pass on genes that predispose offspring to sickle cell anemia.