WiFi calling is no different than 'normal' calling when it comes to minute quotas, if there are any. Any calls placed on WiFi or the normal mobile network count the same as any other call in your plan.
WiFi calling, however, allows you to use mostly any WiFi network (as long as it has internet connectivity and the voice ports used by your carrier aren't blocked) to place/receive a call. This is very helpful when you're out of range from a cell tower, but may have full access to WiFi, which is the case in many buildings where mobile signal might be poor deep inside a building but WiFi is easily accessible. Also, if you're traveling, you can connect to WiFi to make/receive calls as if you were in your home region without incurring in roaming fees. One more case is when the mobile network is poor and/or saturated (for example, at a stadium or amusement park) where you might struggle to get good service; if there you also have WiFi and that's potentially less saturated, you can use that to place your calls.