Because being able to carry around a huge screen is fantastic. Being able to play games or use apps with a split-screen layout when I am sitting on the train is amazing. Folding allows it to fit nicely in your pocket and not be too awkward for quick usages such as responding to a message or paying with your phone.
It seems clear to me that all else being equal having a twice as large screen is a major upside. So it is really just a question of when the downsides (such as price, thickness and durability) shrink enough to tip the scales in favour of folding displays.