mobile - When is using haptic feedback for button presses a good idea?

I've noticed that on Android phones only some button presses result in haptic feedback. For example, pressing any of the 3 main buttons (Back, Home, Multitask) will cause the phone to vibrate. So does any key on the keyboard, and the "All Apps" button on the Home Screen app. But a lot of other buttons don't use vibration at all. When is using vibration on button tap as feedback a good idea?

(I kind of have an idea of the answer but would like to see what other people think)