If you ride your bike hard, whether it's due to commuting or recreation, a lot of miles get put on a bike. Miles on a bike equals wear and tear. The chain gets worn and dirty. Brakes wear down over time. The grease on the components wears off. Rust will also appear.
You may have noticed that your bike doesn't ride as fast or as clean as it did when you first got it. A bike is a machine. Cars are another machine. And machines need to stay lubricated to work correctly. It's recommended that cars get an oil-change every 3,000 - 5,000 miles to keep the components running smoothly. Periodic bike maintenance is also important to keep a bike functioning well and to prolong it's life. A well-working bike is a safe bike. And a safe bike is the best kind to ride.
There are a few things that can be done between tune-ups to keep bikes in better working order - keep the chain lubed and clean, dry immediately after any rain, maintain tire pressure. Learning some DYI maintenance and repair techniques will be handy. Your local bike shop may even offer you a few tips and techniques if you ask. But there's nothing that beats a professional cleaning. Consider scheduling a bike tune up at your local repair shop soon!