I do it to keep track of my position when I scroll a document.

When I'm reading something it helps me find my place again if I get interrupted or need to look up something in a different browser tab. It also makes text more readable when sites do odd things like put medium gray text on a light gray background (for instance HN comments with a score < 1).

Nervous habit, really.

It's the 21st century equivalent of tracing your place on the page with your finger as you silently mouth the words.

Kidding:-) I had a boss who was always highlighting stuff back and forth, and it definitely seems to be some sort of nervous habit more than anything.