GC736 hit the nail on the head.
I work in an office surrounded by huge machines, computers, and a variety of other things. Reception is nonexistent, but Wifi is perfect. With Wifi Calling I can still be reached via cell phone. Any other time and any other phone, I'm lost to the world or stuck using some 3rd party app that doesn't always work and wants me to use an additional phone number (unless you pay for premium services).
And yes, minutes. Some carriers still charge for minutes used. Well if you're on Wifi making calls, you're not touching your minutes (or data) at all.
That being said, Wifi calling was a HUGE selling point for me... HUGE