User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5267262-20130128194345

They seem to be to prevent ships which have the intention of landing. In a space battle, you probably don't have the time/heat capacity to methodically destroy a whole defending fleet and all PDC before attempting to land, so the PDCs attack anything coming close enough to drop infantry off or attack ground targets (wasn't the normandy's scanner working on a resolution of about 15 square metres per pixel at scanning altitude which isn't a brilliant resolution to determine what to shoot at or be accurate about it). They are probably linked to the sensors of the defending fleet giving them a boon to effective range over the attackers too. While theoretically you could sit outside everyone's effective range (unless you are attacking a garden world and respecting council rules of engagement) and try to attack anything that isn't moving, before going to get stuck in with a battle against a defending fleet. That wont work in practise, since soon after the attacker's first barrage passes through the sensors of the defender's fleet, they would know in which direction to travel to find the attacker's fleet. If an attacking fleet does not want to get within effective range of these cannons, then the cannons have at least redirected a space battle to away from the planet where people are in less danger of getting crushed but debris that got caught in the planet's gravitational field.