I believe each base has some core capabilities of defense. Example, I have a Economic Starbase that I can add the "Starbase Defense System" onto. Cost for that is one module. I can also use it to increase the production of my worlds, such as: Finance, Production, Research, etc.
I had to play with it for a bit as I assumed I would need a "Mining Starbase" to increase production output on my planets. Not the case.