Why was the Selective Service Act an important step toward winning World War I?

Lets begin with explaining what the selective service, or draft act was. It was a program in place which young men were drafted into war. They were made to go against their will. If they didn't they would go to jail for draft dodging. Soldiers are only drafted when their aren't enough voluntary ones. This was an important step towards winning WWI because the United States needed more man power and that granted it.