Think about what mastering is, and about how mastering originally worked. You would bring all of your master tapes down to the mastering house, where they would print those songs onto a giant glass disc that became the master press for every vinyl that was going to sold of that album. If the songs changed wildly in level from song to song, it would sound like a bunch of unrelated material and wouldn't be great.
To fix that, the mastering engineer would level balance all the songs to each other and not just to commercial loudness.