Having wifi calling on your home carrier means you can use the data of a cheap international carrier while you're traveling to make calls with your home carrier

Having wifi calling on your home carrier means you can use the data of a cheap international carrier while you're traveling to make calls with your home carrier and not incur roaming charges. This is because your home carrier basically VPN's to your home carrier's network over whatever data connection it has, so it's just using the cheap international data off the second card to make a VOIP call.