Endeavor: Deep Sea · Board game rules
Why is the disk placed on the Team Leader?
Quick answer
The disc on your Team Leader represents your first available action each round, allowing you to activate it and perform its action(s) during Phase 2: Activation.
Details
During setup, you place one disc on your Team Leader's activation circle as part of your starting player materials [p.4 — Player Materials]. Your Team Leader is a Senior Specialist that starts the game with you, and like all Specialists, it needs a disc on its activation circle to be activated during the Activation phase.
In Phase 1c (Reassignment), you can reclaim this disc back to your Staging Area if you have enough Coordination, freeing up your Team Leader for future use [p.7 — Phase 1c: Reassignment]. This is why the rulebook notes that "in the first round, you will have a single disc on your Team Leader, and the capability to reassign one Specialist... so you will be able to reclaim that disc back to your Staging Area" [p.7 — Phase 1c: Reassignment].
The disc essentially "activates" the Specialist, allowing you to use its action(s) on your turn during Phase 2 [p.8 — Taking Turns].