Not everything on Amazon Music can be streamed, and some stuff they just simply don't have in their online library at all. The locker was nice because I had uploaded over a thousand songs that I had from my CD library that Amazon doesn't offer via Amazon Music.
Pushing the button means you will have a copy of those songs until sometime in 2019.
When I log in to view my subscription, I get this notice:
"Your music storage subscription which enables you to import up 250,000 songs will expire on April 8, 2018
After this date you will not be able to renew your subscription or upload more songs. In order to continue to play and download your uploaded songs after your subscription expires, select "Keep my songs." Otherwise, these songs will removed from your library. Amazon music digital purchases will remain securely stored for playback and download."