I pay a little extra for XM Traffic and Weather, which feed in info that the GPS uses for navigation calculations, such as routing around road closures and traffic jams.
I mainly got the XM because I ride a lot in areas that don't have terrestrial radio reception -- places like the wilds of central Nevada, northern British Columbia, the Yukon and Alaska.
And I avoid "dead zones" where they are no strong, clear channels I want to listen to. Excellent disk jockeys (Michael Tearson, Mark Marone, Meg Griffin).
Good talk radio services (CSPAN, POTUS.) I'm irritated that the free Internet listening is gone, and I refuse to pay extra for it, but otherwise I am very glad to have the service.
(I don't think Pandora or Songza come close.) Their news station selection is probably unrivaled, as they simulcast a number of cable news channels, again in a way that I don't think they've distributed to phones.
As I understand it, a significant number of people use satellite radio in areas where there isn't great radio coverage.
Even with a big library of MP3's, I'd rather listen to curated music on a commercial-free XM station.
On days I'm on a long ride (750 miles), it's nice to be able to listen to a baseball game, or an 80's Alt station, or even ESPN Radio, just to have something different.
Sometimes you pick it up to fast-forward through a track, and you have a notification about some social media activity. On a piece of car equipment, you'll find a power button, a keypad to enter a channel number, and buttons to channel-up or channel-down. It's been a while since I sold the car that had Sirius XM in it, but in 2012, there were content exclusives that had no phone equivalents.
Their kids station is fantastic, and almost worth the entire cost if you have kids 2-13.