The -w flag does not turn wifi radio off immediately when you leave range. There is a timeout.

It only does so after x number of minutes (after leaving range of your saved networks).

Privacy flaw in wifi probe requests shares saved SSID's when disconnected (wifi probe requests).

This -w flag is optional, can be combined w/other flags.

EX: If you want wifi off when you leave home/work, but tend to forget to turn it off, -w flag takes care of this w/maintaining whatever other flags you set.