From the tone of your question you may be thinking " if starbases are glorified turrets, why would I wanna spend 10 bucks for that?"
Truth is they are much much more. Each have a vast array of upgradable abilities based on thier factions relative strengths, and the nice thing is that all of them can be upgraded to prevent whatever planet theyre protecting from being destroyed by siege frigates, capital ship and novalith cannons, forcing your opponent to destroy the base to take the planet. and they can all be upgraded to take on huge fleets by themselves.
TEC: Can upgrade for big trade bonuses, frigate production, self-destruct (big red button)
Advent: Can upgrade for super culture strength, fleet immobilization and devastating meteor storms
Vasari: Can upgrade for 100% damage reduction from the front, rapid regeneration and increased damage to structures (including other starbases)
Another nice addition to entrenchment are mines. Theyre a bit expensive, but when well placed, they can do some serious damage to enemy fleets attempting to bypass your defenses