I think there are two factors that drive the purchase. Primarily - it's a status symbol. I am reminded of the scene with Paul Allen's business card...

But also that it's useful when you want a larger display surface in a more portable form. Like watching films on an airplane or reading documents without having to constantly swipe & zoom.

I agree with the other poster(s) that Apple is likely working on a folding model, even though it would sabotage sales of some iPad models.