Their purpose? In regards to who or what?

As far as keeping them away, they are active omnivores and scavengers. Anything that you would do to discourage raccoons would also be helpful for possums.

The primary purpose of an Opossum is to clean up garbage, trash, carrion. They will eat anything, they are omnivours, including another dead Opossum.

Their primary purpose is to reproduce and make more little possums. In order to do that they have to eat to live, and they eat just about anything.

greengardener07, they are also an article of diet and possum grease is touted to be good for arthritis.