87
Spotify Messages can reveal your profile to anyone you've ever shared music with
(www.androidauthority.com)
Icon base by Lorc under CC BY 3.0 with modifications to add a gradient
Generally I think there are two questions: why does Spotify want messages and why would a user want them. The first one seems easy enough to answer: anything that binds a customer more closely to their product and creates switching costs in an otherwise relatively interchangeable product is good from a business perspective.
I could imagine some other use cases as well: Maybe it could be used to allow artists to engage their fans, inserting the platform deeper inbetween those two. I don't really know how merch and ticket sales are currently handled exactly, but a messaging system might also be useful to fully keep users within the app while using those features.