But why paper's ratio is important?

Size is even more important for UI design. If you don't think you need to re-design the layout of your app when the screen doubles in size, you shouldn't be doing UI design.

Moreover, app developers for iOS have to support fewer aspect ratios than developers of Android apps. In theory, this allows for more attention to be given to the UI design for each aspect ratio - rather than having to use a spring-style stretch layout.

If I were to guess, this does look like a new ratio to support, but at least it's just 1 instead of 2.