My inexpert take is that Zen makes a lot of use of contradiction of expectation as a way of waking people up. When my expectation of a situation is violated and I have a WTF moment, I start paying a great deal of attention to the exact details of the situation, and I start questioning all my assumptions.

A good example is the story "A Cup of Tea": [1]

    Nan-in, a Japanese master during the Meiji era (1868-1912), received a
    university professor who came to inquire about Zen.

    Nan-in served tea. He poured his visitor's cup full, and then kept on
    pouring.

    The professor watched the overflow until he no longer could restrain
    himself. "It is overfull. No more will go in!"

    "Like this cup," Nan-in said, "you are full of your own opinions and
    speculations. How can I show you Zen unless you first empty your cup?"

Without being there, I can't know what Kobun intended. (Even if I were I could well not know.) But one way to look at it is him getting people to empty their cups: they thought they knew what would happen, and suddenly they didn't. A joke? [2] A lesson? A metaphor? It almost doesn't matter, because now they're awake.

To me the ultimate purpose of this is to get people to see the flashes of blue sky that the article talks about, those moments of insight that help us see through the useful illusions we collect as a way of getting by in the world.

[1] From http://www.101zenstories.com/index.php?story=1 and I'd encourage interested parties to buy Paul Reps's "Zen Flesh, Zen Bones", a delightful book of similar and related material.

[2] You might also find a useful analogy in the Native American notion of a sacred clown, a heyoka: http://en.wikipedia.org/wiki/Heyoka