How often do you change the air in your tires? How about your brake and radiator fluids? You may need to fill them up or change them once in a while, but you don't have to change them the way you have to change your oil.
Change The Old
You should change the water in your radiator because it gets dirty, and that's one of the reasons to change your oil as well. But there's another, more significant reason that old oil doesn't work as well as new oil.
Inside your car's engine, metal parts are constantly sliding across each other at extremely high speeds. The friction of metal against metal at that speed could generate enough heat to destroy the engine if there weren't oil to lubricate the metal parts and help them slide more easily.
Why Doesn't Old Oil Work?
The long molecules of oil work their way between the metal parts so that moving metal parts never really touch each other. But those long chains don't last forever.
Heat and the motion of the engine break down the long chains into shorter chains and those chains are broken down even more. Old oil is better than no oil and will at least keep the engine from seizing up. But as the molecule chains get shorter, the metal parts are more likely to come in contact with each other causing more wear inside your engine.
When automobile oil changes from the amber color it starts out with to the black color of old oil, that's partly because it's getting dirty, but it's also undergoing chemical changes that prevent it from doing its job as well as it used to.
If you change your oil yourself, save the old oil and take it to a service station or recycling center since used motor oil is very toxic.