If you look at the Add-Remove-Change menu, you'll see it's treated as an additional "language", like French, Italian, or you-name-it, where you can switch back and forth between whichever ones you've installed. Strictly optional, as already pointed out, and just for the fun of it.
The question's already been raised as to how long it will continue to be available for download, so you might as well get it now if you're even the least bit interested. I rather doubt it'll still be available when 5.1 comes out, but who knows?