Yellow marketing pens

I remember when I first got to college how surprised I was that nearly everybody studied by taking yellow marketing pens and making yellow marks to emphasize sections of the text in their textbooks. I'd never seen that before. In my high school -- and, I would presume, in everybody else's -- textbooks were reused, so we were not supposed to make marks in them. Of course, everybody did to some extent, but not marks from actually studying them. So although I could see the purpose, I never picked up the habit. And I'm not sure why everybody else was doing it. Maybe they all got some college preparation instructions that I was sick on that, or something. Even now, though I have underlined sections of books to make them stand out, I don't use the yellow magic marker.