the "more gas in the tank" isn't a good analogy; a much better one would be a "more powerful engine" so that passing another car does not take 5 minutes but is easily done. the spare power is there for purposes like this.
this is still not a very good analogy though.

what one really needs is enough amp power for any occasion that might present itself: sound pressure level peaks in music or movies.
not enough power for one's needs means the peaks will be clipped, distorted.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery