Modern conventional farming relies on a very small number of genetically similar breeds. These animals are bred for high production in a very controlled, high density environment. Consequently, these populations are very inbred. Over time genetic weaknesses arise.
Conventional farming also uses hybrid seeds, which are produced by artificial cross-pollination. Unfortunately, hybrid seeds cannot be saved. The seeds do not reproduce true copies so new seeds must be purchased each year.
The great thing about heirloom seeds and heritage breeds is that they are a “storehouse” of genetic diversity. If you are wondering why genetic diversity is important please read Food Ark by National Geographic.