What other services can a Service Dog do if no leading the blind?

A few months back I ran into this guy who had a Service Dog, which he named “Scruffy”. This dog was not the usual Service Dog, he was not large enough to be a Seeing Eye Dog, and his owner was not blind (he said he suffered severe ADHD). He spent most of the time carrying the dog around to keep it from getting stepped on; it was some toy rat-dog type of breed, I call them ”purse mutts”. Because the dog was classified as a Service Dog, he was able to take it into stores, restaurants, etc. What service was this dog performing? Other than being Seeing Eye Dogs what services can Service Dogs perform, especially ones so small the average alley cat is larger?