Returning Armada is a "vestigial" tech. It used to be a game-ending technology in the original Sins v1.05, but it was severely nerfed when Entrenchment was released. Before the nerf, the phase stabilizers could be set to operate automatically, giving you FREE ships ever couple minutes. So, if you were Vasari, you would makes ~15 phase stabilizers (can put 2 on an asteroid, 3 on a planet) and let the free ships just pour on in. A Vasari player could thus quickly ramp up to having 2300 fleet supply worth of ships.
In multiplayer games, people might sometimes type out: "I have Returning Armada, this game is now over."
Vasari's problem was actually being able to get to the point where you could afford to research it and build all of those phase stabilizers. If a team in a 3v3, 4v4, or 5v5 started out having a Vasari player in a safe eco spot, the guy in the eco spot would often rush to get it up.
Few people ever use it or pay attention to it since the ability was nerfed.