A staging area for mining workers, a dry room is like a locker room but without the clutter and expense of actual lockers. Less common in the modern era, the dry room was a signature element of mines during the late 18th and early 19th century.
Despite being conducted deep within the earth, mining can be a surprisingly damp activity. The rock is stressed and cracked, providing many opportunities for water to seep in from above. The mine is also quite often hot and humid, becoming warmer at increasing depth. Miners get sweaty, dripped on, splashed, and otherwise moistened as they work.
Upon leaving the elevator cage near the top of the mine, the miners would gather in a simple open cavern. They would shuck off their protective coveralls, helmets, and boots. They would each hook these onto a rope with a pulley embedded in the ceiling, the hoist their gear up to the roof to dry. By the time they returned for their next shift, their gear would hopefully be dry and ready for another round. Progressive mines had a peg board or similar system in the dry room for miners to mark themselves in or out of the mine, in case it were suddenly necessary to account for everyone's whereabouts.
I learned about this while touring the Vale Chasm demonstration mine at the Science North Dynamic Earth attraction in Sudbury, Ontario.