Why the Admiralty needed global tracking maps

Was the risk of forgetting distant units a reason naval headquarters used large world maps with ship markers?

Yes. The maps helped staff officers keep the positions and availability of globally dispersed squadrons visible.

An Admiralty anecdote has senior leaders discussing what force to send to a distant region before another officer reminds them that a cruiser squadron is already there. The story illustrates how easily a global fleet's dispositions could escape immediate recollection when leaders were also managing procurement and strategy. Those broad responsibilities differed from the staff officer's specific duty to know where out-of-area forces were operating. This need explains the large maps and movable unit markers used in headquarters. The map was a command tool for maintaining situational awareness across an empire-wide deployment pattern.