An album can often be lost to time for a variety of reasons, whether it was released on an old format to never be re-released, shipped out on an obscure label, or just utter tosh.
Now that we don't buy every album we listen to anymore, it is easy for them to be taken from streaming apps at any time. The choice of what we want to listen to is an illusion under streaming services, as labels can pick and choose which songs make it onto the apps.
Such services are a used as encyclopedia of an artist's career, but music discovery is harmed when labels can choose to hide albums from listeners as they wish. Sure, nearly all artists are now on streaming services, but that doesn't mean that their discographies are complete. These are albums nobody in the world can stream, as of 2019.