[Fyron]

Normally, a ship can only fire on a single target in a round of combat. If you have 6 weapons and the first shot finishes off the target, the other 5 will be unable to fire on new targets. Multi-plex Tracking allows you to fire at multiple targets in a round, so there will be no wasted damage potential. Also, in tactical combat, you can choose to only fire some weapons on the first target and shoot other weapons at different targets.

Point-Defense Cannons are an exception though. They do not have any targetting limits, and your PDC on a ship can hit as many targets as you have PDC. This is if they fire automatically, of course. If you happen to be able to manually target them (such as you have not moved and some targets survivded PDC fire), then you will find that each PDC is treated as a normal weapon for targetting limit purposes for the manual firing.