Climbing shoes are meant to be as tight around the feet as possible. This insures that the feet will not slip inside the climbing shoes, giving the climbers a much better grip and control while climbing and standing on difficult foot holds, and while performing difficult moves such as pulling their body weight with their heels.
Wearing socks can hinder this tightness around the foot by creating an extra layer between the foot and the shoe. This can cause the foot to move slightly inside the shoe, which can reduce the climbers performance by reducing the efficiency of the shoe. The efficiency is reduced because climbing shoes must be as stable as possible on the climbers feet, meaning, if the feet move around inside the shoe, the climber may not be able to exert the required force they need, such as when pulling your weight on your heel. If the climbing shoes are not tight enough, the heel could move inside the shoe, and even come out.
While the climbing shoe should be as tight as possible, this does not mean that they must be painful. There is a limit to how tight a climbing shoe can be without it causing pain. It is important for everyone to try out different climbing shoes and different sizes to find the shoe and size that fits their feet the best.
