Your car battery keeps your car running properly by working in conjunction with the alternator to provide ignition. Even in a diesel automobile which uses no spark for ignition, the engine is designed to not run without a battery.
If your driving conditions are considered harsh - with short trips or extreme weather conditions - your battery may need more frequent replacement. Think about getting stuck at the office or even worse, in the cold, and you'll begin to realize the importance of monitoring your battery's performance.