You get 10 songs you purchased for $10.
or
You get 15 million songs if you pay $10 a month.
Spotify has "off-line" mode for up to 3,333 songs. Which mean you can listen to any of the 3,333 songs you designated without any internet. Save a lot of mobile data.
Look at it this way....
Would there still be a need for buying digital movies on Itunes if paying $9.99 a month for a Netflix-like service that give you access to all the movies and TV shows in the world?
Which is a better choice?
Netflix-like service that has all the movies/tv shows in the world for $9.99 a month (with offline mode for playback without internet connection)
or
use Itunes to download movies and owning the movies
Movies and songs are different. Movies I rarely watch more than a few times in my life. Music I've listened to some songs for decades.