It's a bad habit for me. The OP said "As someone who randomly highlights words and lines when they read" which sounded more like my own tic-like random highlighting than anything useful, like yours.

The odd thing is that I highlight lines haphazardly as well. I hate the popups on Medium because I constantly highlight text without thinking about it. At some point I noticed that it helps me keep my place as my attention bounces around. The selections seem pretty random even to me, but I'm sure there is some sort of subconscious process going on. Maybe it's the same for you? Or not. I have no idea but just thought it was kind of interesting.