I was reading http://starmadepedia.net/wiki/Weapon_Systems and got confused by this section:
The amount of change provided by the slaved system is proportional to the number of modules in the master and slave. For example, if the master has 10 modules and the slave has 5, only 50% of the slave's effect will be applied and this will be the percentage which shows in the Weapons Tab. If there are 10 modules in each, the maximum effect of the slaved system is applied and the weapons panel will show 100%. Support and effect ratios are independent of each other; therefore, if you have 10 master modules, you may have up to 10 support modules and up to 10 effect modules to achieve the maximum effect, for a total of 30 modules (10 master, 10 support and 10 effect).
The wiki talks about mechanics for having modules, but doesn't explain why you need more than one. Does a system work better if it has 10 cannon modules instead of 5? There's no mention of the reasoning behind this.