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.

I know quite a few older people who started to cherish them, as they now have the real estate to finally feel at home on a mobile device they can just easily take with them. Tablets were great, just less portable.

So for quite a relevant segment it seems to become an enabling form factor. There are just people who seem to be basing everything on their own use case. I have neither interest nor need for a fold-able phone. But I can totally get why it is not only interesting, but actually useful to people in different circumstances.