Sometimes, you need to disconnect from your Wi-Fi network in order to test something. And if you’re testing different networks, just turning your Wi-Fi off and on will not do.
Like many of OS X’s bundled menu bar utilities, the Wi-Fi menu has some hidden functionality you can get at by holding the Option when you click it. You’ll see a lot of expanded information, and an option to disconnect will appear right under your current network’s name. You’ll also get some additional Wi-Fi tools, like the option to enabling logging or use the built-in Wireless Diagnostics tools.