It's mainly just to get you in the system should you ever need a loan or a mortgage. My cousin had problems getting a mortgage as she'd never borrowed any money or had a credit card before, she didn't even have a student loan. Therefore banks turned her down as she didn't have a credit rating in the system. The decisions for mortgages etc. are usually based on an automated score card system, so if you're not in the system you may be turned down.
This may seem counterintuitive, but it's the way it is. I purposely got a credit card when I was 18 for this reason and made sure I built a good credit score (though you can see how it leads some people into trouble).
As Dasherman said, if you're happy then don't worry.