Spotify limits audio search to their database/official releases. YouTube allowing users upload they own audio and have “audio only” feature. Integration with YT will add a lot of unreleased. Old or new music and make the app a great tool to discover it.
In comparison with SoundCloud, YT is free and have broader database. It has special channels/users dedicated to different genres used to discover old rare music. SoundCloud pages are more personal and do not have this kind of channels (copyrights issue)