I imagine their thinking (as flawed as it is) is that it frees up local disk space for the user. If you've got a base level MacBook Air with just a 128GB SSD, maybe it seems beneficial to have a 40GB iTunes library stored entirely in the cloud on demand, and suddenly have 30% disk space freed up.
Apple have a similar philosophy with Photos, where the photos on your device are just low resolution versions, with the original hi-resolution versions kept on iCloud, if you choose the Optimize Device Storage setting. [1]
Not a philosophy I agree with. I like to keep my data local so I can use it offline. It's one reason I bought a 2012 MBP - I could get 1TB of storage cheaply, and I can upgrade it to a 1TB SSD myself for half the price of what Apple charges (Apple charges $800 for a 1TB SSD in Australia).
[1] https://support.apple.com/en-au/HT204264