Originally Posted by Retro Grouch

2. They could be "vortex generators". When air hits such an obstruction, it eddys around and can exert a tiny but measurable push against the back of the tire. The amount of force exerted is a function of the surface speed of the tire and the speed of sound so, if you're riding relatively slowly, you'll want longer vortex generators on your tires. When you see TDF riders get a tire change at the start of a climb, they're not be fixing a flat, they're getting tires that have longer vortex generators.