They can be fun, you get a photo plus a short video "for free." Much easier than filming constantly and editing things down later. You can pick a different key frame to display as well, so it can be a good way to fix an awkwardly-timed yawn or such.
I don't use them personally, though, but I'm a curmudgeon about my camera controls ;)
I think Google added something similar in their Pixel camera app.