Nikon d7000 focus points?

Can anyone tell me what's the difference between 9 21 and 39 point AF? They all seem to allow me to select a single AF point where I want on the screen and do exactly the same job.

The numbers speak for themselves

When composing your shots, it is helpful to be able to preset the focus point, so the old "focus, hold, recompose, shoot" process is no longer necessary. This is especially helpful when shooting sports or action.

At one time, I thought that all these focus points were more of a marketing ploy, but now I use those various focus points on every assignment. My camera has 51-focus points and I use each one of them from time to time

I have my Canon dslr cameras set to a single focal point - dead center. When composing a shot I place that point over what I want in focus. When focus is locked I simply recompose the shot to what I want to see and fire.

The single point forces the camera to focus where I want it now where the camera thinks it should be.
Keep in mind the single point doesn't, at least in my cameras, operate in full auto but only in one of the programmed modes, such as Program, shutter, aperture or full manual.

Sometimes all those points can get confusing. You can push the set button in the middle of your controller to select the point in the center. Often it can be faster with handheld shots to focus with the center point and keep that locked in by holding the shutter button half pressed until you shift the camera and take the picture.

With tripod shots, however, having that big collection of focusing points and the ability to select just the one you want can become very helpful.