Well this is a handy feature that most people are not aware of. You might be creating some tutorial or any other thing for that matter and you happen to need some screenshots of your android screen. Believe me, I've seen people use other cameras or phones to take pictures, which gets a big hands down.
The way to take screenshots is to hold down the Back button and then press the Home button. A PNG file will be created in /sdcard/ScreenCapture.