Explanation: When an image is displayed on the camera monitor you can delete it by pushing this button twice.
Afterwards there is no chance to restore it!
When no image is displayed this button has no function.
Together with the exposure compensation button it can be used to format a memory card.
Tips: Well… on a digital camera you need such a button but those who think before shooting need it less frequently ;-).
Alternatively there is a playback menu Delete with additional options to delete several or many pictures in one go.