Why do you need to follow through so much on a tennis or golf swing? After all, the ball is long gone.
Why do you have to honor a money back guarantee with a former customer who is never going to buy from you again (and it's six years later)?
Why do you have to reinvest and retrain an existing employee who needs some guidance when it would just be quicker and easier to hire someone new?
I think the reason is the same in all three cases. It's not because the thing you do at the end of your swing matters. It's because it's a slippery slope.
If you know that the last two inches of your follow through don't matter, then you'll start slowing down at three inches, or even four, and suddenly it does matter. If you draw the line on money back guarantees you'll keep sliding backwards, bit by bit, until it does matter. If you're quick to fire the employee who needs a lot of help, you'll be quicker with those that need just a little, and then, pretty soon, it's a very different place to work, isn't it?
Obsessing about the last inch of follow through ensures that the important parts of what you do get just as much (if not more) commitment.