Barracks are where your Minions call home. They are primarily used by minions to replenish Endurance during the day or night, and are used to store Lockers in order to increase your minion count.
Valet minions will automatically bring exhausted minions to the closest barracks. Barracks are a low-heat room and will not get investigators too excited. Because of this, they are good to build near your base entrance(s).