But why change the oil? Isn't the oil that came with the vehicle good enough?

The answer is no, and here is why...
Over time, all oil breaks down. There are many factors that contribute to the breakdown of the oil in your engine, but heat is the biggest contributing factor. In addition to the breakdown of the oil itself, there are also additives that are put in engine oil. These begin to break down and lose their effectiveness over time. There is also the issue of contamination. Small bits of dust, dirt and debris get into your engine and make the or way into the oil. All these little bits of "stuff" make the oil much less slippery and much less effective at cushioning the moving parts of the engine.
So, how often should you change the oil? Most technicians recommend that you get an oil change every 3000 miles for maximum engine protection. Check your vehicle's owner's manual for the recommendations for your specific vehicle, but be careful. Some manuals recommend an oil change every 7500 miles under certain driving conditions. Make sure your driving meets those conditions before you decide to risk waiting that long for an oil change.