Ok, each time i mention this people act like im an idiot, they do not let any information enter their brains and thus i am unable to explain. Hopefully i can explain in a forum post better than in chat with all the narrow minded, stubborn, probably not very smart kids with very low player numbers shouting that bases dont move and i am a noob.
First of all, no need to discuss the viability of projectile weapons in destroying bases vs energy weapons. I know in most cases energy weapons will do a better job. Projectile weapons only do better on very spread out bases that nullify splash damage and where part of your explosive weapons dont hit any target. I am not trying to convince anyone that a projectile + magnet fleet is the optimal method to kill any base. I am just trying to explain that magnets are indeed a good addition to a projectile ship attacking a base.
When you use a projectile weapon against a base, it will fire the next salvo at a module while the first is still traveling towards the target. Therefore, even if the one traveling is enough to kill the target, your ship will always fire another shot at the soon to be dead target. This is a totally wasted shot. Each module receives at least 1 wasted shot.
With say 35 modules in a base, that is 35 wasted shots. At 4 seconds per shot, this means 140 seconds of a ships worth firepower is lost on wasted shots ! That is at least half its total shooting time (you never get to shoot the whole 300 seconds, you have to move first) Depending if you have 6 weaker ships or fewer stronger ships, this means you lose between 8% and 16% of your firepower. Equiping your ship with irridium magnet 2 will stop this from happening.
However, your other ships might also be firing at this module while the last needed lethal shot already traveling towards it. This will not be 100% prevented with irridium magnets but surely it will be reduced. It is hard to fit a number on this, it also depends on how much you space out your ships. All in all i would say 10-20% of wasted firepower can be prevented by using irridium magnets on base hitter fleets. As an added bonus, the target module will die about 2 seconds faster and thus half the time shoot one less projectile before it dies.