There are a lot of cases where wifi calling is not feasable—assume you have satellite home internet (and accordingly an unreliable ping), or your office’s enterprise network blocks VoIP, or wifi doesn’t reliable cover your house (because MBs provide much better coverage than a single wifi router/AP almost always). These are just a few of the many cases a MB is really useful.
I prefer to take calls over the cellular network because WiFi Calling has been inconsistent for me going nearly a year now. Often times I start my calls on WiFi calling and then switch it off to LTE which provides me with a much more reliable calling experience. Additionally, when VoLTE is turned on, it'll provide a solution for indoor coverage in areas where WiFi Calling simply isn't an option. For folks that live in areas that are only served DSL or satellite, the Magic Box will be extremely useful.