The App has alway been a controller, and it still is too.
When Sonos first brought out iPod docks they were hardwired. About 4 years ago they replaced the wired dock with a wireless version. The latest version of the controller app now allows direct streaming from the phone. So the phone is both a music source (a bit like the way Airplay works) and also a controller for 1/any/all Sonos players in your system. It's all very comprehensive.
As for music content....
You could use the PC as a source library. TBH though, most of us with Sonos use a NAS drive (Network Attached Storage) hooked in to the broadband router. The Sonos player(s) can drag content directly from the NAS. This means you don't have to run a PC 24/7 just to provide the local library for the Sonos system.
What's really nice with Sonos is that there's no special requirements of the NAS. You don't need to buy a special version or run additional firmware or do any messing about. Just set-up the shared folders as you would any network share, then use the Sonos controller app on a PC to point Sonos at the specific shared folders. Using the controller app on the PC just for initial set-up is easier than working off a small mobile phone screen where autocorrect might be constantly dabbling in any typing you have to do. In the end though it's just my personal preference as I setup the systems I sell and install.
The controller app is a free download for iOS, Android, PC and Mac.
