For book-styles: mostly, it’s not having to charge, carry and sync two separate devices. It’s a lot nicer to type on 2-handed than a bar-style phone, especially for those of us with fat fingers, the inner screen is much better for content and games. Outer screen’s aspect ratio matters little as it is mostly for quick things anyway Yes, the flexible screen is a little fragile, but is fairly well protected when folded - moreso than a naked tablet. I’ve shattered the outer screen of my fold and the inner was fine.

I tried carrying a cellular iPad Mini as a phone for a while but kept having to find coats and pants with giant pockets - and I cracked two of them.

I’ve found that a good grip is essential to ease of use on folds though (and I drop it much less).

The phone-to-square ones seem to be most popular with those with pocket space problems, as they fit neatly into small pockets and are less bulk to hang from a lanyard, belt clip etc when you have no pockets.