Advantages of a rooted Android device:
-Wifi Wireless Tether- Use your phone as a wifi router/hotspot for FREE and connect multiple devices. Try Wifi Tether app or Barnacle Wifi app.
-Install a Custom Rom- Google "CyanogenMod" for better understanding. There are all kinds of different Roms out there built for stability or speed or for eye candy.
-Screenshot- With the help of an app called "shoot me" you can take a screenshot of your phone's screen.
-WiiMote- This is one of the fun ones. You can use a Wii remote controller as a controller for your phone. Pair it with a game system emulater (Playstation or Nintendo) and viola! You have yourself a game system in your phone. Check out Wiimote App.
-HDMI full functionality- This is only if you have an HTC Evo 4g. By default, you can only use your phone's hdmi with video captures. But when you have your phone rooted and HDMwIn app installed you unlock the full capability of your phone's HDMI and enable 1:1 mirror imaging of your phone and your HDTV. Pair this up with a game system emulator and the Wiimote app and you have yourself a mini PS3 or Nintendo.
-Overclocking- Turbo boosting your phone.
-nandroid backup- What this does is make a full range back up of your phone, everything from your phones settings to how your homepage was set up. This will come in handy when you've accidentally changed something on your phone and you wished you can turn back time.
Convinced yet? Here are some guides to rooting a phone just in case you're thinking about it.
Step by step guide HERE