Roof racks are popular on cars because it makes them "look sportier" apparently. So it makes a statement that if you carry bikes on your car, then you might be an athletic person. Well, that or because people buy cars the way the dealer sells them. In the US, new cars are stocked on dealer lots - so if you want say a new Jetta, you go to a dealer that has a couple dozen Jettas. Worst case, you find a blue one that happens to be your favorite color with a manual transmission and the nav radio, but they tacked a $200 roof rack onto it and want $500 more now.
Then you haggle it down, tell them you don't even want the roof rack, and pay less than the retail price for the car anyway lol. But sometimes people see it and they're like "oh cool, I've been wanting one of those!"
Who knows, maybe they use it to tie christmas trees to the roof of the car right? XD