Military registration is one way to keep track of personnel who could be conscripted in the Armed Forces in the event of possible conflicts. Some countries like the US and UK have State Guard Units/Reserve Forces made up of volunteers who do part-time military training. Personnel have remained in their Reserve Units for many years doing their services without any government pressure. Because they are trained Reservists, they can be deployed with Regular Units as done in Iraq & elsewhere. To quote the former British PM, Winston Churchill during WW II, "The Volunteer is twice the Citizen". I subscribe to this as I was a Mobilised Volunteer Reservist, effective & proud to be one.