Defense, mostly. But from a practical standpoint it is easier to share maintenance tasks with at least a few other people. It depends on how self-sufficient you want to be. If you're just eating microwave meals that someone delivers to you regularly then, yeah, I suppose you could do it without help. But is that really "sovereignty" if you wouldn't last a fortnight without outside help?
sovereignty isn't about self sufficiency, its about not being someone elses property. the point of the insurance is to create incentives that make it not in the interest of governments to attack you. if that worked than all you would need is the ability to defend yourself from pirates. I could definitely be wrong about this but it seems to me that a single large belt fed machine gun mounted to the deck of your ship would probably be sufficient to dispatch pirates. with that being said, having two or three people on the ship instead of one would be beneficial and there is certainly never ruled out this possibility.