Over time, the belt will become worn and stretched. The belt plays a vital part in the operation of the engine, controlling the opening and closing of the engine’s valves by turning the camshaft and crankshaft. If it fails, it'll cause catastrophic damage to the engine. As a general rule, I recommend changing the belt, pulley and water pump every 50,000 - 60,000 miles or five years (whichever comes first). Most complaints we hear occur after this mileage/timeframe.