Why do American men 18 years or older have to register with the selective service?

American men aged 18 years or older are required to register with the Selective Service to ensure an organized system for conscription in the event of a national emergency or military draft. This requirement helps maintain a ready reserve of individuals who can be called upon if needed for military service. Registration also promotes equity and fairness in national defense obligations, ensuring that all eligible individuals share the responsibility. Failure to register can lead to penalties, including ineligibility for certain government benefits and employment opportunities.