This building allows you to bring in infantry. Improving your barracks opens new types of infantry units and improves their efficiency. The game begins with a first level Barracks. 

On June 22, 1941, the day that Germany attacked the Soviet Union, the field forces of the Red Army numbered 303 divisions and 22 brigades consisting of 4.8 million troops, over half of which were placed along the western Soviet border and in the western military districts.
During the Great Patriotic War, the Red Army rose to 29,574,900 troops.