And then the interviewer is going to ask them, yes but why do we sometimes use flap 10 and sometimes flap 20? A question that is (a) irrelevant, as we do not get a choice, "Computer Seyz No"" and (b) the interviewer is one of the 10 people on the fleet who knows the answer.
Something to do with segment climbs so we can take off with even less thrust than is sensible.
Grizzle over....