What is a ride without dirt, mud and water? No fun. And no fun at all for your bike, especially your bearings, as they suffer through those tough conditions.
Even more impactful than the barrage of contamination is the high-pressure cleaning that often follows. Due to capillary action, the water and dirt is drawn below the seals, into the bearing, and the lubricants get washed out.
The result: rapid corrosion of the raceways leading to constrained motion and, in the worst case, a seized bearing.