Depth-of-Field Preview Button
Explanation: A depth-of-field preview button closes the aperture to the f-stop that is displayed in the viewfinder which will make the viewfinder picture darker but show the same depth of focus as your photo.
At the D7000 the depth-of-field preview button (the lower of the two black buttons next to the lens) is more: A second function key that you can program with the same choice of functions as the Fn button in the custom setting menu f4 Assign preview button.
This gives you the option to reach one of your favourite functions instantly with a movement of the right middle finger (Fn button) and another one with a movement of the ring finger (depth-of-field preview button).
Tips: I don‘t use the depth-of-field preview and consider it a leftover from the analogue era when you had no possibility to check images just after shooting. If needed it is more convenient to check depth of focus on the monitor. But if checking the depth of focus is really important for you (e.g. for close ups) best try it and make up your mind yourself.
My preference is the second option to assign another function in the custom setting menu f4 Assign preview button, please see the explanations and recommendations there. I use it to bring up the first entry of „My Menu“.