Z Fold 4 owner here (soon switching back to a yet undecided slab phone).

>I watch them open it every few minutes

There is something wrong with them as I never find myself doing that. From my experience it tries to eliminate the need for smaller tablets, which it more or less does.

As for the rest. I have relatively small hands and despite the phone being larger and heavier I found it much easier with one hand despite the screen being narrower, that's about it.

As for the "what problem is it solving" part, I can share my experience and that is:

• multitasking - having two reasonably sized windows next to each other is actually quite useful, if you try to troubleshoot some issue at work remotely or taking notes from a website.
• better media consumption on a larger screen, but it's debatable because of the square-ish aspect ratio.
• in-phone photo/video editing is easier and less clunky as there is more space for the elements (e.g. in LumaVision/LightRoom).
• taking notes, sketching with the Pen again easier as there is more room to draw, but I find the crease too distracting.

I also agree with others saying that currently is just a proof of concept, despite the whole phone came a long way from Gen1 to current Gen4.