I discovered this earlier while listening to a podcast in Overcast. watchOS 4 seems to automatically switch to either ‘Now Playing’ for Music or a corresponding Watch app, if you’re on the Watch face.
Pretty nice little feature if you want to be able to control your music right after playing. I’m not sure if this requires iOS 11 or not, though. You can see what I mean in the above video I shot real quick, which demonstrates the behavior between Music and Overcast.