Because you have an enormous library of music, available on any device, instantly without having to download it and transfer it to the device you want to play it on. Also collaborative playlists are pretty sweet, and artist radios to help discover new music based on an artist you like. The vast majority of people don’t want to get their PC out to do all this stuff manually.

Spotify completely stopped me pirating music, never looked back.

You’ll need a broadcasting licence for that, mate.