The Taunt Laser basically forces the unit it is pointing at to attack the Mammoth Tank that is firing the laser. However, this should sometimes not be used, as it taunts any kind of weapon, and once the Mammoth Tank aims it at a Widow, then it is in an unfortunate situation
The taunt aspect is to show how resitant the Mammoth is to weapons which is somewhat stupid as the Mammoth has heavy-type armor and is weak to lasers. This can enrage, enshame, or even amuse enemy commanders depending on which enemy is pointing at the Mammoth and the taunt laser is triggered. This is one of the most questionable stratagies in the entire Command & Conquer universe.