I do it in more or less the same way that I idly thumb the corner of a book I'm reading, or repeatedly open and close a pen while I'm reading code.

It's just something tactile to do while my attention is otherwise occupied.

I do it all the time...just out of habit really but it's one I can never see myself dropping...even in nano on an SSH client :p it makes reading easier for me, I installed flux recently too in order to make the white strain less on my eyes...maybe there's a correlation there?

I don't do this, but I could see someone doing it to keep track of where they are in a large article.

Same reason some people line up the pens on their desk or tap their fingers.

Highlights my place and makes it easier to know where I am while scrolling.