If you are able to use your mobile device as a hotspot, in general you won't need to tether. Old devices required tethering but anything modern you shouldn't have to. When you set up the hotspot you will set the network name and a password for security. As far as bandwidth, you will be using the same data plan as your phone so it will be shared. Typically you will use more bandwidth on a laptop so you will want to keep a close eye on your usage.
The advantage of a jet pack or other types of MiFi are that they support many more options than your typical mobile phone hotspot like 5Ghz data networks and your data won't be interrupted by a phone call or other types of services. You could also have a MiFi from one company and mobile device service from another. This can be beneficial if you are getting a weak (or no) signal from one while the other is able to connect. Of course having two will increase your expense.
In any case, I hope this helps!
