The GSB campaign is a singleplayer 'conquer-the-galaxy' style game, where you build up vast space fleets and kick interstellar ass. Rather than set, pre-determined fleets to battle against, the game utilises the fleets designed by other GSB players to provide an endless variety of opponent, so you will be pitted against real carbon-based intelligence on every step of your path to galactic dominance. The GSB campaign is not a full 4X game, but a contextual 'wrapper' for GSB that introduces new strategic concerns such as crew and cash shortages, repairing ships between battles, the need for construction and repair facilities, and the ability to issue a retreat order mid-battle.
I think a lot of the love for this game here is homerism, as cliffski posts here, is their friend, and they love it for that reason.
Didn't get my $5 worth.