Workarounds for YouTube's terrible, constantly devolving UI always result in someone trying to cash in with something that automates things you can already do, just with more ease of use. The first thing I tried to do was move some watch later videos to a different playlist, so I clicked the checkboxes for the videos, then I clicked on the Save to Playlist button and nothing happened. I thought it was broken. What actually happens is tiny blue 'JOIN' text shows up in the bottom left corner that is extremely easy to miss if your sidebar menu is visible, which mine is, so it overlaps with stuff and makes it look cluttered before disappearing in a couple seconds.

Two actually functional things are the ability to delete videos from playlists in one click with the trash can button, and deleting a group of videos by selecting them, pressing the remove button, then confirming. Useful if you're really bad at removing videos from playlists, but a marginal convenience otherwise. Neither function works from a playlist sidebar when the YouTube player is visible, only from the playlist page.

Maybe it's better if you pay $3 a month/$30 a year, but I have no idea if it actually works, as YouTube breaks these things as often as they come out and I can't test it without paying, and even if it did, I can't really justify paying for saving a few extra clicks.