One of the great things Spotify gives us is the ability to see the exact date that we've added any song, which gives us the ability to go back to a very specific time in our life.
This is the main reason that I came back to Spotify after trying out both Apple and Google music, even though both of those services let you upload any track missing on their platform for personal streaming use, which is the reason I switched from Spotify, but I would rather have the date info over that as the date info gives us a very unique ability (although I really miss being able to upload any track for streaming whenever Spotify doesn't have it).
Let's say that you want to take a trip down memory lane to the summer of 2013, all you have to do is sort after date and voila! Instant nostalgia trip.
The longer Spotify exists, the better this function automatically becomes.
Imagine in 5, 10 or even 20 years, going back to the month that you met your wife/husband/best friend and being able to listen to the specific tracks (even the ones that you forgot about) that you listened to in the period of meeting them.
Our brain reacts very strongly to music, so much so that nursing homes around the world have begun using music for people with dementia to make them remember stuff. It's one of the most intense nostalgia factors we have.
BUT, what completely ruins this is the fact that whenever you create a new playlist or want to manually merge/combine several playlists by moving tracks, you lose this information and therefore the ability to go back to specific times of your life, unless, you keep every single playlist you've ever made...
So in other words, whenever you create a playlist, you better be sure that you never want to change something about it or merge two playlists with subgenres to a more general genre, to make your library more organized, because if you do so, you lose this info.
My suggestion is that Spotify does something like this:
That way you can always go back in time to that specific moment in your life while moving tracks around and create new playlists as you want!