From T-Mobiles Web Site:
Wi-Fi Calling allows customers to make and receive calls and texts (SMS & MMS) over a Wi-Fi connection. Using Wi-Fi Calling is easy – customers won’t even know they’re making a Wi-Fi call. Simply connect to an available Wi-Fi network with a Wi-Fi Calling enabled smartphone and continue to make a call or send a text as you would over a cellular connection. No separate app, log-in or number is needed.
Their web site indicates that you can receive calls as well as make calls on your cell phone using the wifi connection.
Thus, if you don't have good cellular coverage at home but have a good wifi connection, the WiFi calling capability should help.
I remember a number of years ago, Nextel phones had this capability but it never caught with others.
I think the reason was because the cell phone companies were still charging by the minute and making a wifi call seemed like it should have been free since the user was providing the connection.