Most internet giants strike deals with major labels explicitly with music services in mind, but not Facebook. It just reached a worldwide agreement with Universal Music Group that allows the group's music to be used across Facebook, Instagram and even Oculus VR experiences. This will let you (legally) upload videos with Universal artists' music and "personalize" your music experience. In theory, you could drop some Kanye West or Tove Lo into your Instagram clips without worrying about a possible takedown.
This is also just the start. Universal notes that it's the "foundation" for a roadmap, and that the two will "experiment" to bring "new music-based products" to Facebook's platforms. This doesn't guarantee that you'll see a full-on music service (Facebook would need to sign other major labels before that's even an option), but it's clear this is about a lot more than sprucing up the soundtrack for your vacation highlight reel.
To some extent, this is about fending off copyright issues. Facebook likely isn't interested in dealing with copyright takedown notices, and labels have long accused internet companies of not doing enough to curb piracy. This puts Facebook in the clear. However, it could easily help Facebook draw people in. If you knew your favorite artist made a song available to use on Facebook but not on Twitter, where would you go to share something related to that tune? It could make Facebook more compelling if you're a music fan, even if Facebook needs additional deals to have the selection you crave.