What is the purpose of these things (from Cats and Dogs)

I think it's an evil vet clinic sort of thing. You sell treats that make pets sick, then the pets come back to be treated. Or if a pet comes in for a checkup, you could give them the treat, thus making more money off treating them. You also might want to make your pets sick in order to treat them and increase your vet skill. Some people might also want to make them sick for storytelling purposes.