Does a VPN used on a smartphone with Wi-Fi disabled (mobile data only enabled) provide any sort of protection?

I’ve never completely understood this, but I think the answer would probably be “no,” although I’m not sure. Usually when I leave the house I turn off wifi and just use mobile data (this is a habit from my pre-VPN days), although I guess I should probably just keep it on since using strange Wi-Fi with a VPN is ok (unless...