Let's say you're playing a large or larger map. Defending your far-flung colonies can be difficult right? They're far away so your spaceships can't reach them in time to defend it against attack.
So instead, just build intensive farms in those colonies. Obviously this works better with super-breeder. Once the pop is high enough, taking those colonies with enough troop transports becomes cost-prohibitive for the AI.
Combine this with starbase fortifications and you don't need spaceships at all. You can just mass colonies and population, and fortified starbases, and laugh at the futile AI attempts to hurt you.
