A difference operation returns the items present in the first list and absent from the second. Order matters: A minus B is not B minus A.
The two directions answer different questions
What do they have that we lack? What do we have that they lack? The first is an opportunity list, the second a defensive one, and running only one of them tells half the story.
The classic application
Your keyword set against a competitor's. The terms they rank for and you do not are the gap — the single most actionable output in competitor research.
Normalise first, always
Difference compares exact strings. Any inconsistency in case, spacing or trailing slashes produces phantom gaps that waste real time.
Try it: List Difference (Keyword Gap Finder) on SeoWolf's Notepad.