Best case, someone finds a quote that expresses a rationality idea that I agree with but couldn't articulate as eloquently as the quote. This is particularly nice when it comes from an unexpected source; when I see good rationality coming from places I didn't expect, it's evidence that the corresponding ideas are good ideas rather than just, say, ideas popular on LW.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.