First, with iTunes on a PC you need to turn on "iCloud Music Library" with an Apple Music subscription (if you haven't already).
See the 'On your PC with iTunes for Windows' section in:
Turn on Sync Library with Apple Music - Apple Support
With an Apple Music subscription, you only lease the music to play on Apple authorized devices for as long as you maintain your subscription. The music is copy (DRM) protected and you cannot change the format or burn it to a CD (for example). The "purpose" of the subscription is that you get access to literally millions of songs for a nominal monthly fee. You can still purchase music from the iTunes Store (with or without an Apple Music subscription) in which case you own that music and can do whatever you want with it for as long as you want within the copyright limitations.