When you back button focus, you are no longer using the shutter button to autofocus. Look at the picture below to see the button I use:
1. It's easier to use the focus-recompose technique.
With back button focus, I can focus on my subject, recompose the shot, and take as many pictures as I want without having to continually go back to my subject to grab focus in between each and every shot.
However, when the shutter button controls autofocus, I have to focus, recompose, take one picture, then aim at the subject again, focus, recompose, take one more picture, etc. It's tedious, time consuming, and easy to miss a shot this way.
2. My images are sharper and I have more keepers.
Sometimes that split second between pressing the shutter half way to pressing it all the way is enough of a time lapse to get an out of focus picture. With back button focus, I can simultaneously press the focus button and the shutter button so there's no time lapse at all. This helps tremendously with moving subjects (kids!).
3. It doesn't matter if something gets in your way.
Have you ever been in the middle of taking a picture when someone or something enters the picture area while you're shooting? Your autofocus gets completely thrown off for a moment. This doesn't happen when you BBF.
4. The D7000 has a sensitive shutter button.
Compared to my old Canon Rebel XSI, the D7000's shutter is extremely sensitive. When I tried to press the shutter button half way to focus, I kept accidentally pressing it all the way and it would fire off about 6 shots! My husband jokingly said the D7000 shoots like an automatic weapon and it's true!
How do you switch to BBF?
This will differ depending on your camera so please check your manual. But on my Nikon D7000, I had to do the following:
CUSTOM SETTINGS MENU
f5: ASSIGN AE-L/AF-L BUTTON
When you first switch to BBF, it takes a little getting used to using your thumb, but it quickly becomes second nature. Let me know what you think if you give it a try! Or if you already BBF, do you like it better?