To answer the original question, Sprint is depended on for anything that requires communication when you (and your cell phone isn't around). Remote lock/unlock, etc. Why Sprint? No clue. I know Verizon and GM are very friendly and usually AT&T is friendly with anyone who comes with large sums of cash. That leaves Sprint and T-Mo. And T-Mo was in process of getting gobbled up by AT&T back when the decision was made. Officially, no clue.
Hotspot may require that as well but I don't remember 100% on that. Don't quote me on that one.