Presumably so that way you can start playing a video, go into the background, and have the audio keep playing.
It plays sounds when you interact with the app (liking a post, etc.)
Re:Facebook iOS App Battery Explanation
by soccerdave on Thursday October 22, 2015 @09:47PM
(#10436363)
An app does not have to request the special permission to play background audio in order to play sounds.
Re:Facebook iOS App Battery Explanation
by TazeTSchnitzel on Thursday October 22, 2015 @07:56PM
(#10435995)
Audio calls, maybe?
handled by the Messenger app these days. Not sure why they would need it in the Facebook app
Edit: Here's a link to the apple docs that show apps have to declare what tasks they will perform in the background.
https://developer.apple.com/library/ios/documentation/iPhone...