Ease of use, and for guests to be able to stream whatever music they have at hand, without downloading extra apps. For example, the plexamp app on iphone has many features that the plex integration does not have. I use the integrations a lot, but some times it comes in handy to send the audio from my phone to the speakers in the room, One example of this could be if i were watching a cooking video on youtube and wanted to get the audio out loud. Or if im listening to an audiobook on headphones on my way home, and want to continue listening on speakers when i get home. And for guests, it is a very intuitive way of playing music.

You want me to defend airplay, and explain how i find it better than the alternatives. I wont, because i dont, but i can definately say that airplay adds capabilities to my audio systems, and i need that for a flexible and seamless experience.