As of yesterday I had an app using mostly static sizes to fit an iphone 5 screen (320 points width). It was working fine in iphone 6 as well thanks to the system scaling up automatically.
Then I decided to add a retina hd launch image and everything became a lot smaller on iphone 6. So I decided to modify my whole application to use dynamic sizes and fonts so that it would fit the two iphone types the same way. And now it is working quite good.
But after these few hours of extra work, I keep asking myself the same question... what was the point? Why would you want to use dynamic sizes that fit both iphones when you can make it work for iphone 5 and let the system scale up automatically?
the use is: more screen estate. You can fit more on the screen.. at the least images or maps could be bigger while buttons retain there size -- they normally don't need to be bigger :) same for the keyboard.
you don't just get everything scaled but you can decide what is scaled and what isnt