The wheels on your skateboard act as a buffer between you and the ground and determine how rough your ride will be. The smaller hard park/street wheels will carry less speed but accelerate quickly which is what you want for transition and hitting gaps, rails, etc. The big gummy longboard wheels carry tons of speed and grip the road way more than the small hard wheels, but there are freeride and hybrid longboard wheels that are on the harder side for the sake of easier slides and better park performance.