Originally Posted by QrafTee

"Rooting" is essentially the Android's version of "jailbreaking." Although it's already an open platform, there are certain areas of the phone that is locked to prevent tampering and withhold the "integrity" to the manufacturer's "standards." Rooting the phone allows one to flash the ROM, run a different OS, run a different UI build, run a custom build, unlock features locked away for one reason or another, and/or even install fixes by third party developers instead of waiting for official updates by the manufacturer.

That's my understanding anyway, I may be wrong.