On solid objects at least (like jewelry) it dislodges particles of grime / dirt. https://en.m.wikipedia.org/wiki/Ultrasonic_cleaning. No idea if this also works on soft tissue.

Edit: the Wikipedia page above says "the ultrasonic action is relatively benign to living tissue but can cause discomfort and skin irritation.". So maybe it was just a gimmick. Ultrasound cleaning was fairly new at the time, so maybe it sounded modern.

Oh, ultrasonics can definitely harm flesh if focused. Yes, I am the guy who tested it with a room humidifier. I knew it would hurt, I did it anyway.