They are because they keep stressed corals from being overtaken by algae that grows on them and competes with them for sunlight :( Also the bites that parrotfish do while feeding on coral zooxanthellae actually keeps them healthy. Kinda like pruning.
The only reason I know is because I applied for an internship under a guy who studied that kind of stuff and I wanted to impress him with how much I knew about his work
ended up not getting a spot at his lab but hey, I learned stuff!