We had XM in our 2012 S P85 until we traded it in last summer, and have it activated in our 2017 S and 2018 X.
The primary reason is to have access to live sports & national news, especially in our X that lacks AM radio.
And with the subscription, we can also use our smartphones to stream XM over Bluetooth for additional channels, such as getting our team's sports broadcasts when the satellite stream is using the opposing team announcers.
Other than sports and national news, we don't use XM for anything else.