Because I get the benefit of a smaller phone and a larger one in a single device along with much better multitasking. I can't exactly fit an iPad in my pocket, can I? Not to mention a foldable iPhone will match/exceed an iPad Mini and still weigh a hell of a lot less.
Not everything is about watching a basic video on a larger screen; it's about running apps designed for an iPad or even the Mac on the go. Being able to do the equivalent of full video/photo editing like you can do now with the more professional apps on a Mac with a device that fits in your pocket is really nice. But now you can just quickly fold the screen and put it in your pocket and treat it like any other regular slab phone.
Ignoring the creases, it's the ability to have this in my pocket.
I really don't get why some just don't understand the benefits. It's like trying to explain the benefits of a car to someone from 300 years ago; once you use it, you could never go back.