I've been using the Windows Phone App for Desktop since Windows 8 and Windows Phone 8. I prefer this app because once I check each artist whose music I want to sync, any time I add more music from one of those artists, all I have to do is connect the phone and open the Phone app on the desktop and the new music is automatically synced to the phone.
I last used this app in January, but now it crashes each time my phone is connected to the computer. Even after uninstalling and reinstalling the app, the same thing happens.
The Phone Companion app just says, "To listen when you're offline, move the song files to your phone (just plug your phone into your PC to get started)." I guess they now want us to use file explorer to copy or drag the music files into the music folder. It's too bad they never migrated the syncing function from Zune into the Groove app if they want Groove to be the primary source of music. The Zune (both software and hardware) is the reason I started using Windows Phone in the first place.