One is a diversification of offspring. This allows the offspring to get a random assortment of genes from their parents, and it may be very optimal, leading to greater success. Also, if a disease is able to wipe one of the rabbits out, it isn't a guarantee that the entire population will also be wiped out since they don't have the same genetic makeup that would lead to the perishing of all since some may have more disease-resilient genes.
