By Jailbreaking you can turn the device into what it should have been before apple restricted it.
Apart from customisation, you can apply tweaks and other utilities that allow you to exploit the device and install anything you want (within reason)
It is simple to do and opens up a massive range of possibilities.
It doesnt really void your warranty as if you have a problem it is a simple matter to do a restore before taking it to an apple store to get it fixed