They have a purpose, but it's not diving.
A family member was married in the DR last winter. My side of the family are fish. The other side, not so much. The idea of using a snorkel scared the hell out of them - good people, just not water people.
I pointed out that these were available not too far from the resort where we were trapped/staying, and they agreed to go and check them out. A bunch of us went out with a local operator. They were tied to a giant buoy so they couldn't go too deep (15 ft to the keel), and the DM in charge just pointed them where he wanted them to go. These were in bad shape, as far as I could tell, had almost no propulsion/steering capability.
It didn't really matter. In calm water, for non-divers, they were a bit of a hoot. It was a fun way to meet more of the "other side of the family", but I've checked it off my list, and I've no interest in using one ever again.