What is the purpose of using a cylinder cap during storage?

The purpose of using a cylinder cap during storage is primarily to protect the valve from damage. The valve is a critical component of the cylinder that controls the release of the gas inside. If the valve is damaged, it can lead to leaks, potentially causing safety hazards such as fire or asphyxiation from escaping gases. The cap serves as a physical barrier, shielding the valve from external impacts, dust, and contamination, thereby ensuring the cylinder remains safe and functional until it is needed. While the other options may have their purposes in different contexts, they do not address the primary role of the cylinder cap, which is crucial for valve protection.