A credible source: AndroidPolice
tl;dr: Google brought in an option (by default enabled!), which is a checkbox reading "Scanning always available" in WiFi -> Advanced.
The general purpose: "To improve location accuracy and for other purposes, Google and other apps may scan for nearby networks, even when Wi-Fi is off."
To improve location accuracy
by geolocating your IP, matching it with GPS, probably seeing if you use a proxy
for other purposes
IDK, peeking into your data maybe?
even when Wi-Fi is off
why is there a Wi-Fi switch then?
Given, that Google has not a reputation for invading the privacy of users a bit too much, that is/was the general motive.
As said; you can turn this somewhat useless feature off (hopefully); and get some more battery life on your phone.
P.S. The fact that this option is not explicitly stated to the user when he/she begins to use the phone, and that this option is significantly hidden, concerns me.