Tim Cook's Apple created a music streaming service for the exact same reason Steve Jobs' Apple created the iTMS. To accomplish the same goal though requires different moves in 2017 than in 2003. With regards to streaming services, that's where they growth is.  That's where the new revenue is.  iTMS sales have leveled off and Apple, having been surpassed by companies like Netflix and Spotify, is no longer a leader digital distribution of music and movies/TV shows.
Why does Apple want to create original content?  Because they are a hardware company and want to sell more 

 TVs.  Why would a consumer buy an 

TV when they can get the same content on other services (Netflix, Amazon, Hulu, etc.,) which are already available on everything from TVs to video game consoles to set top boxes? If Apple makes some great original content and it's only available on their service and the only way to get their service on your TV is with an  

TV then you've given consumers a compelling reason to buy an 

TV.  Original content is a must for any successful network TV channel, cable TV channel, and streaming video 'channel' (service).