It's a source of income for the single-player campaign, and is tied into the core mechanics. Kenway's Fleet is accessible via the ship's cabin on the Jackdaw.
This resource metagame has become a staple part of the Assassin's Creed series letting us dabble in sending off our assassins to complete missions for rewards and XP.
With Black Flag your captured fleet can bring in additional revenue which is a must to afford all those upgrades for Edward Kenway and his ship the Jackdaw. Apparently when friends are online the Kenway's Fleet missions are easier to work, so it now has a social element which unfortunately makes it server dependent apparently.
When Uplay servers are in trouble then the mini-game becomes unavailable, and completely sealed off from you if there's no Online Pass attached to your Uplay account. I thought we all agreed passes were silly?
Assassin's Creed IV: Black Flag released on Xbox 360 and PS3 this week, with PC due November 22nd/25th.