I get it, if you use them. However, I see a ton of people buying roof racks just for the look. Why? Personally, I am all for function over form. Unfortunately, I don't see the point of adding a roof rack if you don't/won't ever use it--or you'll use it two or three times per year. Why not just take it off?
ETA: specifically, I'm talking about cars that don't have factory rails. I'm talking about the Thule (or other brand) ones that you buy completely separate and mount them to the roof. Not just adding cross bars to factory rails.