I don't think why the airstone works is known, but the effectiveness doesn't seem in dispute. One theory I've read is that when the bubbles break they fling the biofilm in every which direction, some of which winds up getting flung outside of the tank. I suspect that by keeping the surface film mechanically disrupted the biofilm material either settles out onto the surface or is trapped in the filter (if you have one). in my non-filtered shrimp-only tank the airstone keeps the surface clear and I do seem to notice some amount of "flingage" around the bubbles but couldn't say whether that's how a meaningful amount of surface film is disposed of.
To the point about skimmers and shrimp safety, I have seen shrimp get swirled around in the airstone bubbles without apparent ill effect. I wouldn't say they enjoy it since they don't seem to actively seek out free bubble rides. This is different from the way the shrimp do seem to seek out sitting in a water flow over a surface: River run - Fireplace aquarium
(see the 'shrimp on a shelf' video). Weird but they do this all the time. I think there is some kind of "swim upstream" instinct in these guys.