Why? Because even if you turn off wi-fi there's a way to remotely get in even if the software says it's off. All it takes is your IP and phone number and a hacker can hack your wi-fi although you think it's off. Best idea is to remove the hardware completely or use a prewifi vintage phone.
Yeah, see - You must be in a pretty suburban area where cell service has good coverage, and where it doesn't cost a mint to go over on your data. Since I am @home most of the time, or at any number of client places where I know the wiffy network, most of my data usage is over WIFI
And all I have to do is get off the valley floor and cell service goes away. Plenty of places around here do not get cell coverage - When I go out to see several of my friends, I use WIFI calling to stay connected while at their place.
WIFI is dang near more important than cell around here.
And no, I have found no condition where the net services transmit without those services being on. I have monitored traffic on purpose over my network and have never detected a single blip from my phone if the wifi is turned off.