https://www.cnet.com/news/amazon-launches-ad-supported-music-service-for-alexa-devices/

Amazon's Prime membership service, which costs $119 annually, includes access to a library of more than 2 million songs and thousands of stations and playlists. Amazon Music Unlimited, which starts at $7.99 a month, offers over 50 million songs.

With music being one of the primary uses for smart speakers, like the Alexa-powered Echo devices, it makes sense for Amazon to offer more ways to give Echo owners access to songs. After starting folks off on the limited library available for free, Amazon could convince some of them to transition over to a paid service, such as Prime or Music Unlimited.