Welcome to the Evo section, lol
Rooting is.... awesome. Ever heard of jailbreaking an iPhone? Rooting is the Android equivalent. Practically speaking, it's the exact same thing: giving you, the user, administrative permissions to your phone, something a lot of us think should be ours in the first place.
A rooted phone is very customizeable. You aren't forced to use the stock setup. Furthermore, particularly with the Evo, a lot of the modifications actually improve your phone's performance, most notably:
battery life
doubled frame rate for smoother animations
increased touch-tracking sensitivity
ability to throttle your processor
ability to apply skins and themes to your UI
and much more
Because the Evo has a huge rooting community, there's lots more stuff available for you to try once you are rooted. To start learning about rooting, check out the sticky thread in this subforum:
http://androidforums.com/evo-4g-all...ot-guide-updated-21-15-bst-aug-31-2010-a.html
Feel free to ask questions if any info you read is confusing.