It's highly likely you'll need to have your iDevice connected to the car via USB for this to work. Don't underestimate what a pain in the neck it is to fish your phone out of your purse/pocket every time you hop into the car and hook it up via a USB cable that may or may not be conveniently lying around. It's just disgusting that CarPlay isn't inherently a WiFi thing.
Strange, I automatically take my phone out of my pocket nearly every time I get in any car anyway, plugging it into my car is just a quick step from there. The iPhones have been too big to be comfortable sitting since the demise of the iPhone 5/SE size.