Many years ago, a friend let me borrow one of his digital cameras (a Canon 1D Mark IIn). My first question was "why doesn't this thing focus?". He had it set up for back button focus and proceeded to tell my why I should be doing this too.

The idea is that you change a setting in one of the "control functions" (you can do the same thing with a Nikon also, but there is probably different lingo), and one of the buttons on the back now tells the camera to focus, rather than the shutter button doing focus on the half press. There are a long list of good reasons to do this, not the least of which being that people will never ask to borrow your camera more than once.

1. You can focus and then freely recompose without keeping your finger on the shutter button.
2. You can focus and then shoot much more quickly.