Smaller overall size of the phone. If the folded thickness is less than a double of the current slabs, and the unfolded size is bigger than them, doesn’t it make sense to you?
With the “current” technology, we could make phones that have 9” screens and insane battery life (The ratio of screen power usage and battery density isn’t 1:1. Think tablets and their insanely good battery life) but it would be impractical, but if you fold them, you get that exact thing.