This is a quick way to delete images you no longer want when you’re in playback mode. It will give you a warning, though, before actually deleting a photo – you’ll need to press this button a second time.
If you’ve set the D7200 up to shoot JPEG and RAW files simultaneously, these will be displayed as a single image in playback mode, and if you delete this image you are in fact deleting both the JPEG and the RAW files.
If you want to erase all images from the card, it’s good practice to use the Format command in the Setup menu – this erases the card completely, getting rid of sub-folders and any other data that may be lingering from previous shooting sessions or indeed other cameras if you’ve used that memory card in more than one.