There are only a few rare use cases where it makes sense. It's not real WiFi. They're just using an onboard hot spot to connect to cell towers, which is exactly what your phone's hot spot would do for you, and you're already paying for that. Here are a few use cases where it might make sense...
1. You don't have an unlimited data plan on your cell phone
2. Money is of no concern for you. It's easier than firing up your phone's hot spot and every device connecting to it, so it's up to you to decide whether that convenience is worth $20/mo or whatever it is for you.
3. If your vehicle hot spot gets better coverage and streaming reliability than your phone's hot spot. You can always try it out for a month if you're struggling to get consistent reliability from your phone. The vehicle one is likely a more powerful hot spot than the one on your phone.
4. You don't get great coverage everywhere you go with your phone provider, and the vehicle uses someone else's towers. i.e. the vehicle uses AT&T towers which are more reliable in places you go than Verizon, which is who your phone is through. Easy to google which vehicles use which company's towers.
If money is truly no object and you want the best internet and reliability, get a Starlink Mini with a roam plan. You can easily mount it to the roof of your car or even just lay it on the dash and you'll get fast and reliable 100+ mbps service pretty much anywhere.