Because it's a larger screen in a smaller package? Yeah I know it's seems really obvious, although it's not as dramatic because unfolded it's probably not that much taller than today's largest smartphones. Flip phones didn't have foldable screens, I would really not compare the Flip with a flip phone in any way except simply that it has a hinge and closes, but they are vastly different phones/types.
But you shouldn't be so close minded and just throw the entire category out. We have yet to see what different form factors and functionality oem's can explore. Personally I think that foldable phones lend themselves better to unfolding to tablets, if I can fit a tablet into my pocket that's a game changer.
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Don't forget AR glasses though, yeah I know that's a long shot as well. But instead of looking at your phone or watch all day, if you could look at the world and get all that extra information, notifications, etc without missing a step, that could be very powerful. The problem with wearables is they screen is way too small, especially something like a watch, to be used as anything other than an accessory to a phone, although some functions are possible like phone calls, music, etc., it would still be pretty hard to ONLY use a smartwatch all of the time. Not impossible, but not what most consumers would do IMO.
Personally I think foldable phones will definitely take off, with the only differentiator being price, they need to come down in price. But it's not like the $1k+ smartphone market is hurting either. I'll see your flop call and say that my call will be that foldables will be very popular but that popularity will rise only as prices fall and technology makes folding phones better (improved reliability, thinner, multi-folds, external screens, different form factors, etc.)