From what I understand (and I may be wrong since I haven't actually used it), it would simplify the process of creating something like the ubiquitus search box Google uses in Google Maps. Where it sends the string you type through a tree-type process parsing it into structured meaningful data. Of course, that's just the first example of a use-case I thought of when I saw it. I'm sure it's a lot more powerful than that.

It's a very simple parser generator. You can quickly create a grammar to parse whatever code you want to compile (or otherwise transform) and then manipulate it from Ruby. It simplifies compiler creation considerably.