A 15V TVS diode will have a standoff voltage of 15V and a breakdown voltage that is 1V or so higher. I would think that it would make sense for such a diode to be connected in parallel with a 14.4V automotive supply. If the supply is reversed, then a unidirectional diode presents a forward voltage drop of 0.6V, thereby taking out the fuse. OTOH, a bidirectional diode would afford no reverse polarity protection. An overvoltage, such as an alternator load dump, would cause either diode to go S/C and blow the fuse.