I do it, too. I rarely read any text without subconsciously marking the text while reading it. Might be a tool for me (ADHD) to make it easier not to lose track - I don't know.

But regardless of why people do it and while I agree that it's probably something very specific not a lot of users do, I refuse to believe that anyone actually uses those select->popup-> share features, ever. Often the little pop-up even blocks the text above it which is just insanely bad UX imo.

Sites should never mess with core functionality without asking (scrolling, selection, tab/keyboard navigation, hijacking common shortcuts/right click, clipboard, history, etc).

I believe someone came up with that idea a decade+ ago and people just want it on their site to add value without actually checking if anyone uses it.