Incongruity implies that some aspect of the situation is out of place.
o/" One of these things is not like the others. o/"
But it is not just any incongruity that contributes to humor, but an incongruity that has redeeming factors. Not just the wrong word, but a word which provides a double-entendre. And as our sense of humor gets more and more refined, the subtlety of the incongruity must be greater.
Incongurity is certainly an attribute which is ideal for educational purposes; it calls into play our ability to recognize patterns, to isolate the offending aspect, while recognizing the warped way in which it does fit. Just what we need to understand certain behaviors are proper, while others are not. What we need to help us detect deception, where most of the situation is precisely what it seems, but certain aspects are subtlely out of line with reality, or at least our mental model of it.