Back-button focus: why?

[M_Six]

I started using it when I got my macro lens and my Kenko 1.4x TC. With both the macro and the TC the focus may not always be right where I want it, so I adjust the focus manually just a bit. If I then press the shutter button, it refocuses back to where it was. I could set the lens to MF, but if my next shot will be AF only, that becomes a pain. By using the BBF, I can get a focus close and then nudge the MF ring and shoot without having to worry about the lens readjusting itself. Try focus stacking with the shutter button set to AF. Doesn't work well.

[Busted Knuckles]

I think both these ideas are helpful depending on the type of photo you do.