What is the purpose of caps or suitable devices on turret nozzles?

The purpose of caps or suitable devices on turret nozzles is to prevent moisture or foreign materials from entering the nozzle. This is crucial for maintaining the integrity and functionality of the extinguishing system. If moisture or other debris were to enter the nozzle, it could potentially impair the discharge of the dry chemical agent, leading to ineffective fire suppression. Keeping the nozzle clear and protected ensures that the system operates as intended when it's needed.

In the context of the options provided, enhancing visibility during use, increasing spray distance, or reducing the weight of the nozzle do not directly relate to the primary functions of caps or devices on turret nozzles. Protection from contaminants is the key reason for their implementation.