Cause you're a gangsta and wanna be fly with those blin bling rims that make your car slow as shit. Seriously tho, the larger the wheel the more rotational mass you have, which eats power. Getting a smaller rim cuts mass, thus improving your power to the ground. Sometimes its been said that having a shorter tire (by running a + sized rim and tire) lowers the amount of sideways shift in the tire, improving traction in the twisties. All trade offs, but go with 15's.