Some cylinder valve designs (e.g., O-ring) have a metal-to-metal seal between the gland nut and the valve body to prevent external leakage past the threads. The sealing works till there is no external damage to the gland nut. Any impact on the gland nut may lead to leakage through the threads, which the users generally struggle to arrest.
Secondary protection is provided by an O-ring in case the metallic seal is compromised
Gland nut O-ring is introduced in the valve body below the gland nut threads to prevent leakage.