They don't need to add value for the end user if they can control the scarce resource (content) by buying it up and providing more lucrative monetisation options for creators.

Their play is a hybrid of a streaming service and an adtech giant, and I imagine they're keeping their options open to see whether it pays to keep both sides of that model open or to pick one. Either way, one of the reasons I don't like what they're doing is because I can see it working.

Besides all that, there is some value to having all of your audio in one app.

This completely ignores the fact that hosting costs don't really matter. If Spotify can make more money on new subscriptions than they paid some content creator for exclusivity then it's worth it to them. And if the content creator gets more money by selling their exclusivity to some platform than they got for ads then it's worth it to them too.