What do double key locks specifically prevent?

Double key locks are specifically designed to enhance security by requiring a key from both sides of the door to unlock it. This feature is particularly effective in preventing access when glass is broken, as an intruder cannot simply reach through the broken glass to open the door from the inside. By necessitating that a key be used for unlocking, double key locks ensure that the door remains secure even if other vulnerabilities exist, such as broken glass. This makes them an important security feature in environments where maintaining control over access is critical.