This saves you from guessing how much storage space your local music is eating up.
That's especially important if you let the app cache music as it plays. The APK is signed by Google and upgrades your existing app.
When you buy albums or songs on the Google Play Store, that music will be immediately added to your Google Play Music account as well as being available for download.
But you may want to buy music from other stores like Amazon, rip it from your own discs, or just download music that isn’t available in Google Play Music and get it in your library. Songs you try to upload will be “matched” if possible.
Update 8/7/14 #1: Quite a few of you experienced issues with album art after updating to version 5.6.1616P.
It’s a free feature that allows you to access your music collection from anywhere.
If you’re considering buying an album that isn’t available for streaming in Google Play Music All Access, you may want to consider buying it on the Google Play Music Store.
This has properly cleared "My Music" on my Apple Music i OS apps, etc.
However, while the music files do not show up now on SONOS in the "My Music" section of Apple Music, the list of Artists does, and the list of Albums does.