Almost every mobile home has a crawl space where the plumbing, some wiring, insulation, and pier supports live. Even if you have a "ground-set" home, there is still a crawl space.
Keeping out critters is essential to keep this space in good condition. Without good crawl space covers, all kinds of pests will get in and set up shop.
Ruined insulation, droppings, and dead animals are some hazards you may find, but this could even include chewed wires or baby animals!