My guess would be the idea is to shift listeners from white noise that Spotify pays royalties for toward white noise that Spotify owns. The hidden factor is ads that don't actually play for Spotify subscribers. The general idea behind paying for an ad-free experience is that royalties and revenue are either paid by subscription fees or by advertisers. The free tier gets ads and the paid tier doesn't. If Spotify shifted users to content they don't pay royalties for, then users wouldn't notice the difference, but behind the scenes Spotify can claim the royalties and ad revenue for itself rather than pay them out to a third party.

For example, if Spotify replaces the podcaster who makes $18k month in royalties with Spotify's own white noise podcast, then the immediate effect is that $18k/month doesn't leave Spotify's pockets.