There is a lot of truth to the answer given by bru! For most folks it's just about appearance, and filling the wheel well does indeed look "better" IMHO. However there is a more important aspect and that is keeping the overall diameter of the wheel to spec when mounting low profile tires. The overall diameter of the wheel shrinks when lower profile rubber is mounted and the difference has to made up with a larger rim. Keeping the overall diameter to spec ensures that the wheel sensors are happy which control your ABS and Tracion Controls not to mention speedometer. Nobody denies the performance enhancements given by lower profile tires and that's why we are seeing more and more of them on domestic cars. Of course, performace rated tires are also mounted and they by nature are more expensive and have a shorter tread life. If you want to play, you have to pay!