Yes, a complete and fully featured bookmark that won't disappear. It costs me absolutely nothing, and is ready to go when and if I need it. Why wouldn't I do that if I could ?

Yeah, this is what I do, too. Also keep in mind, though, that you'll need to exfil your fork if you think it might disappear at all from Github. They yank the original repo and all of its forks for example if they get a DMCA notice. Years ago, I set up automation that pulls repos that I've forked, so for me, the act of forking ensures I get a copy of the code imported into my personal Gitlab instance on my home LAN.

It makes perfect sense. This isn't what I've seen some people do, though.

I use stars for this, but I guess one could use forks as kind of "super stars".

Yes, I'll almost certainly star a project if I forked it. I certainly star way more than I fork, I assume most people would do the same.