Weapon Racks were introduced in the Alpha 17 update as part of the Armed Guards system. It requires the Armoury research before it can be placed. When Tazer Rollout is fully researched, guards will come to the Armoury to get tazers from the weapon rack.
When a riot occurs, prisoners will rush to the Armoury to get guns from the Weapon Rack
The weapon rack also shows a sniper rifle, which cannot be used yet.