The main benefit is convenience. Let’s assume that your home or work site does not have good cellular, but it has a good WiFi connection. Your phone connects to WiFi, and will allow you to make calls through that. This means the terrible cellular signal is of no consequence to you as long as you have WiFi.
Another benefit is if you decided to travel out of the country. Rather than paying higher rates for making a cellular call, you can make all of your calls back home when connected to WiFi (such as your hotel). Since it does not cost the carrier to offer WiFi calling, there is no need to charge for the privilege of using the service you already pay for.