"Apple says users should try to keep Wi-Fi and Bluetooth turned on for the best experience on an iOS device".
In an area with wifi networks that you cannot join, the phone will continue trying to join ad infinitum. Seems to prevent downloading emails via cellular on my phone. Once I completely disable wifi, I get a bolus of emails.
Even if you don't our can't connect to those wifi hotspots, it will still use them for location info.