Although Android is open source it still does not give you full control over your device. By "rooting" your Android device it provides you with root access which basically means you get full control of your device, you become the Administrator and not a user with limited access. You paid for the phone so why would you not be allowed to do what you want with it?
Advantages of Rooting
- You can get performance boosts as it allows you to tweak the OS and install custom ROMs.Some devices do not support newer versions of Android but thanks to people building and customizing ROMs you may be able to run newer Android OS versions.
- Rooting your device removes operating system limitations, such as overclocking your device or being able to use the LED flash on your phone as a torch.
- Some devices have hardware which support certain features but the software does not allow it. By rooting your device you can make use of these features - such as live wallpapers.
- With Android devices you cannot install applications onto your SD card which is really annoying - rooting your device will let you to use Apps2SD which will allow you to install applications on your SD card!
- A lot of applications will not work unless your device is rooted, this can become extremely frustrating as there are many awesome applications out there which require your device to be rooted.. Rooting your device also allows you to make use of applications from other builds as the different Android builds have different stuff, your friend may have something which you don't and you may be able to get it - if your device is rooted only though.
- Tethering!! Having a rooted device allows you to make use of your WiFi or Bluetooth to share the data connection on your phone with your laptop or PC - very useful for if you are on the move and need to quickly check something and would prefer to do it on your PC with a bigger screen or if you need to make use of a certain application on your laptop / PC which requires internet access.
With the release of Froyo a lot of the reasons / need to root your device has become useless although there are still benefits to rooting your device. Rooting your device will void your warranty and it is dangerous but you can always change back to your original default OS. Remember - do so at your own risk!
If you would like to root your device I recommend you google for a tutorial / guide on how to root your specific device. Loads of users on multiple forums have posted guide / tutorials on how to root your device as well as provide links to the software you will need etc.
Good luck and enjoy,