The timing belt synchronizes the camshaft and crankshaft to allow the valves to close and open at the right time during the operation of the engine. Replacing the timing belt along with the water pump included is a safety measure that needs to be done regularly every 75,000-100,000 miles to prevent engine damage before it fails completely. If you are experiencing weird noises from the engine, low oil pressure or worse the car wont start at all then your timing components may need replacing.