Because the firmware will wipe your device, make a backup before proceeding. And while it is not an official release from Samsung, installing it will not void your warranty. What you will find is that the UI has been changed to the one used on the Samsung Galaxy S4 and the Samsung Galaxy Note 3 with a new tabbed interface for the Settings application.
The launcher and widgets remain the same as the ones used previously on the Galaxy S III, and secure business application Samsung KNOX has yet to be included in the test builds of Android 4.3, although it is expected in the final build. If you can't wait for the official release, click on the sourcelink to install the leaked Android 4.3 update for the GT-I9300 version of the Samsung Galaxy S III.
source: SamMobile via AndroidAuthority