When connected to a wifi? network, the phone can send the call over the internet rather than the cellular network. This benefits the carrier, but can also benefit you as the subscriber. Wifi calling will allow you to use a much stronger wifi signal than the weaker cellular signal. Some older houses may even be hard to get a good cellular signal, so wifi calling will be the best solution.
Another advantage of wifi calling is when traveling internationally. When you return to your hotel, and assuming you have a good enough speed, you can make calls back home using the hotel’s internet connection. This means you are not using the cellular network to make the call. This is free to you. Just tell everyone that you would spend a certain amount of time every day making return calls.