Recommended Oil Change Times

Hi. This is not a problem I'm having, but just a general question. I know it is recommended to change your oil every 3,500 miles or 3 months. Whichever comes first. The part I don't get is with the 3 months, what if you've only driven the car a couple times within the 3 months, or if the car just sits for that time and the engine isn't even being run? The 3,500 miles I can understand because the oil is being used. So basically my question is why would you need to change the oil if it's hardly being used or not being used at all?