To preserve battery. I don't know about iPhones, but on my Android phone, Wi-Fi is certainly the largest battery hog, if I leave Wi-Fi location scanning on.
I get around 1.5 days normally, less than a day if I use wifi and BT.
Just checked again to make sure. Wifi 0%, Bluetooth 0%
Right now wifi is at 8%, and I used it for maybe 15 minutes this morning, that's it.
Motorola Moto X Play, Android 6.0.1, for what it's worth.
As far as I know, Marshmallow has a battery drain issue, but Motorola (now Lenovo) does not seem to be interested in updating their software anymore.