Anecdote time: I do this, and I know it's weird. You just have to look around to see that almost nobody else does it (and it drives people I'm around nuts with the constant clicking).

I think it started in like 1998 when I was a kid using ancient slow browsers on tiny fuzzy monitors over dialup speed internet. I'd be in the middle of reading an article/forum/etc and something would make me lose my place, like a picture (finally) loading above, or trying to scroll at 7 fps and the page jumping way past where I was (I never trusted pgdn), and it would be quite the chore to find my place again. So I started selecting around where I was reading, especially before scrolling, just so I can easily find my place again and it just became a habit.

Now I'm using a giant WQHD monitor at 144 Hz using the fastest page renderer in the world (webrender), so I really don't have those excuses anymore, yet the habit remains. At this point it's probably more ADD and habit than anything else; if I'm not selecting text, I'm fidgeting with something, a pen, usb drive, etc.