They're investing heavily in podcasts because podcasts are far, far more profitable than music. If they can get people used to (and hooked) on listening to podcasts (any podcasts) through Spotify then all that money spent on popular podcasters will be worth it (in the end).

I'm sure Spotify would love it if they could stop streaming music entirely and just focus on podcasts. Streaming music costs them a ton of money and overhead (bureaucracy associated with keeping track of and paying artists globally with bazillions of laws and regulations and fees to navigate) whereas podcasts just cost bandwidth.