XBMC has a lot of cool add-ons, but not all of the cool possible additions come directly through official channels. If you want to get off the beaten path, Fusion can help make it easier to install unofficial (and untested) add-ons.
Fusion provides a list of third-party repositories that you can add directly from XBMC itself over the web. Normally, you'd have to find a ZIP file for the add-on, download it to your machine, and install it manually. If you're using XBMC on a home theatre PC with a regular remote, this could be a pain.
It's worth noting that Fusion is not an official source from XBMC and, in fact, XBMC advises against using it due to the potential for untested software to cause problems for your installation. Much like jailbreaking or rooting your phone, you're taking your tech into your own hands if you go down this route, but if you know what you're doing, there are some good add-ons out there. Check out MakeUseOf's article at the source link for installation instructions.
How To Set Up Fusion Installer On XBMC [MakeUseOf]