Indoor MOBILE hotspot - help me understand

I'm having a time squaring the usefulness of an indoor MOBILE hotspot that provides cell coverage to the house/small surrounding area. Whats the benefit/use case? It obviously needs a high speed internet connection, so there has to be one present. If there's high speed internet present, there's likely WIFI. If there's WIFI I can make WIFI phone calls. If I can make WIFI phone calls, why would i need the T-Mobile/Helium hotspot?

Is the coverage of the indoor hotspot really that superb to a router and some WIFI extenders?