When you are using 3G/4G, and not connected to Wifi, your phone still checks all the Wifi accesspoints continuously. When you know for yourself you don't connect to an accesspoint on that location, there is no point in having your phone search for one. You can save on battery there.

A personal experience from me (anecdotal)... I was using Wifi from my cable provider that is available all over town. But that means when going into town, I am jumping from one accesspoint to another, sometimes with good and bad connections. My experience was that a battery charge didn't last that long. Disabling that accesspoint on my phone made the batterycharge last much longer.

For tiny values of huge