Right now, when an artist releases promotional singles for an upcoming album, they have to relase the track as a seperate single.
In Apple Music or YouTube Music, promotional singles are dealt with differently. The album is fully put onto the service, but only the promotional singles are avaiable for streaming. The user is still able to save the full album to their library, essentially pre-saving the album.
This allows songs that are scrobbled to last.fm to have proper tags, as the music is tagged with the name of the proper album (as opposed to the promotional single).
It is also way more organized and allows the artist page and the user's music library to be less cluttered. And when the entire album comes out, the user does not have to unsave the old versions and resave the new versions. It's all automatic if it's categorized under an album in the first place.