Look, it is completely irrelevant what "tech" setting the game is in - there are very few reasons for putting any sort of defense on the ground if it could be put in space instead.
The foremost reason for doing it would be to repel a ground invasion: Active planet based defenses makes excellent sense when the assumption is that any enemy wants to take the planet mostly intact with an invasion force. It should be abundantly clear in Sins that this is not the case: eradicating the population is not a choice of last resort, it is the choice of first resort - you don't send in an invasion force in Sins, you destroy the population centers and keep up blasting the planet until every last vestige of organize civilian opposition has been destroyed, and you do this from as distant a point in space as you can get away with.
A second reason could be if a ground based installation would be considerably more powerful than a space based one despite having further to travel and the athmosphere to pass through before getting to its target. It is a heck of a lot of work for any science fiction writer to come up with a good enough excuse for this one, but it can be done. The typical approach is "a lot of missiles on the planet and really bad point defense on the part of the invader and the invader, for one reason or another, not vaporizing the launching spots immediately when noticed - probably because you caught him in really low orbit directly over the launchers". Sure, it could work, but it doesn't sound like a plan to count on saving you from any enemy who prefers to hit the planet from extreme range. Flight time is a bitch.
The only thing that makes general sense regardless of science fiction setting is to place defenses in space - shorter range to their targets (unless you have your defenses hugging the planet), less problems for line of sight weaponry (assuming that any of your weapons are line of sight for purposes of guidance), less issues with collateral damage when the attacker starts wiping them out and to have shelters/city forcefields/whatever on the planet - things that actually help your people to survive..... and Sins already has all that, because that's really all that makes sense when the goal of any attacker is to wipe out the organised population, not to conquer it.
Interestingly enough, it is entirely possible that the planets, asteroids &etc in Sins already have significant ground based defenses to ward off invasion (in the minds of the designers of the Sins universe), but none of us would know since we are only extended the option to nuke 'em till they glow, then polish the world to perfection with our main batteries.