I'm fairly sure that Hyph means that if you have a weapon graphic where the left side of the turret is a huge long gun part, and the right hand side is a glowy red thing...
If you put two of those on, say, an Eagle - they're going to look weird, because both of the weapon-graphics will have their asymmetry facing in the same direction, and that tends to look wrong (especially on symmetrical ships!)
As such, it should (when the proposed weapon file option is set) mirror the weapon sprite using the ship's centre-line.
This would let people make weapons that blend in with a ship hull, and use them on both sides of a ship. As to -why- you would want to do that, well, it could mean for a total-conversion mod that a missile variant of a ship could have huge missile bays built into the wings, an energy ship would have several barrels, but the base ship would have only the stubs of the wing drawn on, and would probably look weird without this.
As it stands this isn't possible, because you'd get two wings both pointing left if you tried this.