I rarely use Spotlight any more, since QuickSilver does such a fabulous job of finding files on my computer. But unlike Quicksilver, Spotlight searches an underused file property (or should I say Info) that users may access by highlighting a file and pressing the CMD-I. Notice the first field is called Spotlight Comments? It never even occured to me what that field is for, but you can add metadata (tags) to any file so that it becomes searchable via Spotlight.
One possible use for this... Let's say that you like to test out a lot of freeware Mac applications, but you end up having so many small programs in your Applications folder that you forget what they even do. By entering a description into the Spotlight Comments field, you not only get a description, but searchable data. So months after installing Audacity, you think "hmm, don't I have a freeware audio editor somewhere? Now where did I put it?"
So now you can launch a program without even knowing its name :)