Ahh, the magic term... cross-platform development! It is the current hype thing. Because before the era of smart phones and tablets, the Windows platform, OSX and Linux were basically the only markets, where an individual game developer could publish games to. Game consoles were accessible only to huge game development companies with a lot of financial background.
Even these days, it is almost impossible to publish a game for an individual on some platforms. But new markets have arisen and new development tools such as Monkey are available now. Today it is more and more important to target several platforms with your game.
Why miss out on the money that can be made and the extra exposure for your game? There are so many markets to cater to, such as Apple AppStore, Android Market, and others, that it would be silly to throw away the opportunity to publish on them and rake in the extra cash. So, a cross-platform development tool such as Monkey is worth its weight in gold, and you will realize this very soon.