Because they dont really do anything it was decided that some time ago that each Admiral in the marines would be given a specific task and authority over a specific branch of the navy, along with a system of checks and balances for each-other
The Fleet Admiral (who is still an NPC) Is given the task of maintaining balance and order between the other three branches of the navy and authority over all admirals and naval crews, tasked with internal operations and intelligence
Admiral Nishki D Rock (Nikz200) Is given authority over all Vice admirals and their closest subordinates and is tasked with the defense of marine and world government bases and security of its land and water-space and will provide them with missions
Admiral Payne (TheBlackLuffy) Is given authority over the seven shichibukai captains and subsequently their crews and is tasked with the claiming of pirate territories, dismantling of pirate under-structure and capture of powerful pirates, and will provide the shichibukai with missions
Admiral Darnel Warshack (The One) Is given authority over the bulk of the navy at large as well as given the ability to take authority in matters of internal investigations as he is tasked with exposing and convicting corrupted and unlawful marines from the navy and has been granted permission command higher forces from the world government to aid in his investigations (such as cipher pol or other wg officers)
Each Admiral is also ordered to hand pick a small squad of individuals from the marines to serve as their personal crews and will be commissioned funds to enhance or acquire a ship for suitable travel (those users are aloud to allot 5 slots to other users to make members for their crews, must be captain rank or lower, each admiral's crew may also create or customize their ship as they see fit)
Similarly, the admiral's characters are now charged with providing missions or tasks for their subordinates, to perpetuate activity.