Back in the album days, an artist needed a song or two or three on the album that would would have wide appeal and be easy to quickly like, so that they might become hits. These would get played on mainstream radio, and based on them people would buy the album. They would listen to that album for those songs, but because it was a pain to skip around on an album they would listen to the whole album.
Artists could put deeper, more complex songs on the album. Songs that people might not instantly like. Songs that people only come to appreciate after hearing many times. Songs that eventually become people's favorite songs on those albums.
With streaming, a lot of people will just jump to the newest hit from another artist instead of listening to the less immediately accessible songs from the previous artist whose hit they just checked out.