Animals are prone to get run over by cars. The road kill is often left on the side of the road. Opossums eat the roadkill cleaning up the roads we travel by.
Opossums eat Insects that can spread diseases like yellow fever, plagues, and malaria. This Lowers the chance other living beings can get these diseases.
Rabies is a disease that affects animals, but not opossums. Opossums are one of the animals that are less likely to get rabies. If an opossum got rabies the opossums immune system helps terminate the rabies because the opossums have a stronger immune system.
Opossums also help keep the population of other animals down. For example rats, skunks, and raccoons population would be much higher than it is now. Skunks and raccoons can carry rabies and rats can invade your home.