Suppose you are searching for the use of a particular paradigm in a scientific study. You find one article that uses it in GS, and to find more you then click on "Cited by n." At this point it would be very convenient to search within the results to try and narrow them to only those who cited the article and used the same paradigm, as indicated by your keywords.

Also still waiting on this much needed feature:
blogoscoped.com/archive/2006-0 ...