Military starbases are your key to defender advantage. In Civilization, the defender gets faster healing on their territory, their cities can bombard enemy armies, etc, the defender has an advantage.
In Galactic Civ you need a military starbase to achieve this. It's a "home turf" advantage. Early on in the game they boost the effectiveness of your ships by an ENORMOUS amount. This will become less significant with time unless you upgrade your bases. If you're playing a turtle strategy, say, you're playing as the Altarians and going for a tech victory, military starbases can literally win you the game.
I'm happy with their effectiveness, what annoys me, however, is trying to defend a huge empire with them. I end up needing a dozen of them. I think there should be more ways to extend the range of, specifically, military bases.