It served as a rest and sleeping facility for the Captain, separate from his regular private quarters. Occupation of the ready room was required by regulation when the ship was underway in times of war, when the Captain must be immediately available to the bridge at any hour in case of emergency.
The concept of a ready room has been popular in recent years in the TV show 'Star Trek'. In Star Trek The Next Generation, Star Trek Voyager and Deep Space Nine, the ready room served as an office of the captain or commanding officer of a starship. Here, they could work or hold meetings in private yet still be a moment's notice away from the bridge of their ship.