Wheel bearings support the vehicle's weight and allow the wheels to rotate smoothly, decreasing friction. Over time, the constant weight of the vehicle can break down bearings, seals, and grease, leading to failure before the intended 150,000 mile lifespan. Common causes of early failure include contamination or loss of grease from leaking seals, or misadjustment. A noisy bearing indicates contamination and should be replaced, as loss of a wheel while driving could be dangerous. Distinguishing bearing noise from other issues like CV joints requires noting whether noise occurs only when turning or at certain speeds.