Airplay from within an app used to be different than airplay the entire screen.

This was invaluable especially from the photo app, where sharing the entire screen shows everything.

Starting airplay from within the photo app puts it into a form of slideshow, where I can scroll all the photos and videos and select one, one at a time for it to appear on the screen instead.

It is a much better experience for viewers on the screen not seeing the entire device screen. If you switch to another app, what's on screen doesn't get affected.