Poll: Do you select text while reading?
When people see that I highlight text while I'm reading, they always want to know what practical reason it serves.  The simplest answer is that it kind of helps me track the text, but it's less than that.  It also isn't quite just a nervous habit.  The only description that makes sense to me is that it feels like a physical connection to the page.  More like spinning a pen, or rolling an object in your hand while you think about it.  Like the mouse is my fingers and I'm just sort of fiddling with the page, like flicking through page edges of a book just for the feel of it.