Actually what the rule states is this “If two or more weapons are of the same type, they may be fired together. One attack roll is made for all weapons being fired but each additional weapon adds +1 per damage dice to the final damage total.” So you have a choice you can roll to hit with each weapon individually doing potentially three rolls of 2d6 each having to deal with armor individually or you can make a single roll and add a +1 per additional damage dice but only have to deal armor once.
Example if the target has 4 armor and you fire individually hitting twice with a average roll for 2d6 of 7 each time 6 points will penetrate the armor or if you combine the fire and hit with the same average of 7 you do 7+4-4 or 7 points penetrating the armor. This combined attack becomes even more effective with weapons with more dice and vs targets with more armor. But by the same token it’s less advantageous if the weapon has the AP trait. Using the same example but with a rail gun instead the individual fire total would be 14 since the rail gun has a AP value of 4 and the combined damage would only be 11.
