It's to emphasize what you're doing in the app over the app itself. For instance, in the case of an app that handles audiovisual content, the content ends up do

It's to emphasize what you're doing in the app over the app itself. For instance, in the case of an app that handles audiovisual content, the content ends up dominating the focus while the app itself fades into the background. It gets out of the way and lets the user focus on the task at hand. Quite a contrast from the approach of many third-party iOS developers, which often put a lot of energy into creating a comparatively flashy custom UI/UX for their their apps, often as a method of branding.