You would need to read the papers to be sure. Look for Arias and Gupta c. 2012, of interested. My understanding is that typical ASP only works with programs that can be reduced to a finite set of propositions. Goal-directed execution does not require grounding, so you can do it with answer set programs that have infinite structures like lists, trees, etc., you consult only relevant rules (I think), the returned set includes only relevant terms, and it can be structured as a tree-shaped justification without much additional overhead. It may also have something to do with its ability to reason in the presence of certain kinds of loops, but I'm not sure if goal-direction actually matters, there, or if all ASP suspend can do that.