Advantage:
- standalone device, so you can still use your phone separately and not using the same data quota
- has its own battery. No or minimal display so the battery can last quite long.
- as it is usually a router, it usually can support more devices connected to it.
- it might get better reception since the unit and antenna configuration is solely for the Hotspot functionality
I used to use one of those. But not anymore
- phone battery life is getting good enough
- more convenient to just use my phone and don't have to carry an extra device
- data plans in my country is quite cheap that I don't mind using my main quota for tethering