NOTE: This extension is NOT the Yahtzee game. It only "enhances" the game by adding true randomness to the dice, but it isn't necessary to play the game. There's a separate app (that might eventually include this extension) for the game, or you can just go to yahtzee.silentmatt.com. If you don't care if the dice are truly random or just "close enough" there's no reason to install the extension.
The extension is necessary to access random.org directly (i.e. without a proxy on your own domain) due to the XMLHttpRequest same-origin policy.