They're vortex generators and they are technically illegal for UCF racing so nobody likes to talk about them.

As the tire rotates, those little nubs generate air eddies that swirl around and generate a tiny but measurable push against the back of the tire. The amount of push is a function of the speed of sound so slower speeds require longer vortex generators to maximize effectiveness. If you watch the Tour de France on television you'll frequently see bunches of riders getting wheel changes in the mountain stages. The teams say they are puncture changes because purely aerodynamic aids are against the rules but the riders are really changing to different vortex generators.

Most recreational riders would benefit from keeping those nubs as long as possible.