It depends on the user, that's for sure. A folding "book" type both by price and concept I think is very specific. I see it as having a normal phone and an iPad mini in a single device and, at least in the case of Samsung, it knows how to give software that lives up to both uses. Folded is a normal Samsung phone, but unfolded has all the features of Samsung tablets, with window management and very powerful multitasking.
If Apple releases a foldable iPhone, the least I ask is that when it unfolds we have iPadOS running, while folded it has iOS like any iPhone. If you don't have that, it would be a complete disappointment to me.
In my case, which I communicate with clients mainly via message/email, I like it because I can save and upload files to my Google Drive, where I have all my clients" files and I don't depend on anything to save documents anywhere. Having a big screen also makes it more comfortable to read long documents.
For me, a folding book style is not the same as a shell style phone. I find the former more useful than the latter.
The folding "shell" phone I see as useful for people who want to have a screen of more than 6.5," but do not want to always carry that size in their pocket, since they become uncomfortable for certain people and pants. With the exterior displays that carry the latest Razr and Flip 7, they have a perfect display for viewing messages and responding to short messages and seeing specific things without deploying it. For normal use, you deploy it and have a large device in your hand, but compact in your pocket.
However, for me, the foldable "book" phone is the most useful, since you have a normal phone, which you can use like a regular phone, and deployed you have a "mini tablet," with the advantages of having a device large enough to be able to do certain productive tasks. Obviously, it does not replace a laptop, and perhaps not a 12/14" tablet, but it is capable of replacing an 8/10" tablet, especially if it has the same software as manufacturer tablets, as is the case with Samsung.