If every offspring has the same genetics, they are vulnerable to all the same things.
Example:
6 rabbits are born
3 have long legs
3 have short legs
When foxes attack the 3 rabbits with long legs survive because they can run faster
20 roaches are placed in a dish
Pesticide is sprayed
If they are all the same. They all die or all survive.
If they are different genetically there is a better chance that some will survive and pass on the gene