Personally I've never had a use for a tablet- I literally bought an iPad once and returned it the next day because it didn't serve a purpose for me. Of course that's not true for some people.
But I do sometimes have a use for the large screen on my phone, and I would definitely have a use for a smaller (one-hand-sized) phone screen, so having both of those available in one device appeals to me.
I'm on iPhone now and I've "regressed" back to a relatively normal phone size with the iPhone 7. I almost got a Plus but I had an iPhone 6 before that and got comfortable with the smaller size. I'd probably be fine with the SE at this point, but since Apple has dropped that form factor I'm not going backwards.
So yes, I can see a place for a folding device when one needs to carry both a phone and a tablet, but for me at least, I can move to a phablet for that.