Why would someone pay for a WiFi hotspot that you can't take out of the car when you can have a smartphone that fits in your pocket?
Because kids. And better reception. We're on a shared data plan. Road trips can use a lot. If I can have them use wifi in my truck from an unlimited plan, then I don't have to worry they'll burn through our shared data and we will *all* be on 2G speeds until the bill resets. Also, I've found the hotspot in my Silverado gets better reception in low/spotty reception areas then my phone does.
And also for dashcams with cloud support!!! I had the Onstar LTE trial for the few 3 months I had the truck. My Blackvue dashcam burned through 4Gb in about 2 weeks on a road trip. With this, I can still let my dashcam upload and still have my shared data plan for my phones.