SiriusXM is dying, if not already dead. Why do I need satellite radio when I can just listen to a podcast, or preload a bunch of music on my phone? Or just stream? 10 years ago, you could still do these things, but it was harder or very expensive. Now, you just open an app on your unlimited data phone, plug in the aux cable, and start driving.
So this purchase is about Sirius re-entering the market. I think they'll pivot pandora to be more self-serve and they'll merge the good things people liked about sirius (comedy, shock jocks) with the good things people liked about pandora (choice, freemium)