I bought a foldable phone (Galaxy Flip) for the gimmick, but having used it now for a half a year, it's really grown on me, and I've actually found the folding features to be quite invaluable.
One nicety of the folding screen is in being able to quickly see notifications without opening the phone (and thereby being tempted to browse/procrastinate)
with my folded phone, I can quickly hold it in my palm and glance at the time and any notifications.
Another benefit of the smaller form factor is being easier to fit the phone into my pockets
as phone sizes became larger, I was progressively having harder time finding clothes with large enough pockets for my phone. With the folding phone, I can comfortably fit the phone into my pocket, while still having a larger screen when I need it. It also makes it easier to take my phone with me while running, as I can fold it and place it into my running pouch
a larger phone might fit, but the volume would no doubt be uncomfortable against my body as I move.
Bear in mind, I'm referring here to a folding model which folds into a small square and thus has a smaller area
even I don't fully see the benefits of the larger-form-factor folding phones.
Wow... reminds me of lord of the flies, although a much happier ending for the children.