A large inactive fleet is of limited value without crews who can be recalled to operate it. Reserves can also guard against militarism because major expansion requires the activation of civilians with lives and professions outside the service. That makes mobilization more visible to society and gives the force a constituency less dependent on a permanent military career. Reservists retain livelihoods beyond the service, which can make them an additional institutional check when political leaders ask the armed forces to act.
Notable Point · 1:50:46-1:53:17
Why Starfleet needs a reserve system
Starfleet's apparent lack of trained reservists is both a mobilization weakness and a missed civilian check on an entirely professional force.