No legitimate reason. Part habit, part compulsion, part tic. It's basically doodling with the selection highlight.

Edit: don't need to share all that, I think.

To me it has become a sign that i am bored of what i am reading and should just skip to the story's punchline. Or sometimes abandon it altogether.

I often select text so that I have an easy visual reference when I'm scrolling down. I don't want to scroll too far!