How important is an Xbox 360 hard drive?

I just recently purchased an Xbox 360 Arcade and am trying to figure out some things about getting a hard drive for it. Someone told me you need a hard drive for online, why is that

This entry was posted on January 31st, 2010 and is filed under Games Questions, XBOX 360. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response or Trackback from your own site.