The ruling in the book explains the rationale. I'm not saying I agree with it, but it's right there.
The scenario they propose is 6 seconds remaining in the half with a stopped clock. The QB spikes the ball, which runs 1 second off the clock. Now, a field goal try should run the clock out without the other team getting the ball back with 1 second.
The interesting part is that the ruling states that the penalty is intentional ground, with a 10 yard penalty, a loss of down, and a 10 second runoff. The runoff was not included in this instance as it should have been.