Hoses and belts ensure the engine runs smoothly. The belt helps to keep the engine running at a steady speed, so it can start and run at the right speed.
The belt that drives the alternator and water pump is an important component of your vehicle. The alternator charges your battery, and when it's working properly, you will be able to start your car without having to crank a manual hand-pull starter.
The water pump keeps your engine cool by circulating coolant through it. Without a functioning water pump, you would overheat very quickly on hot days or long drive distances because the engine could not keep itself at an appropriate temperature range.
Finally, belts also drive power steering pumps; if this belt were to break while driving down the road, you would lose power steering capability as well as battery charging ability until you could repair or replace it.