What is a primary function of the valve cap in a tire assembly?

The primary function of the valve cap in a tire assembly is to prevent moisture entry. This is essential because the valve itself, which is responsible for maintaining air pressure in the tire, can be compromised if moisture gets inside. Moisture can lead to corrosion and deterioration of the valve stem, potentially causing air leaks. By sealing the valve when it is not in use, the valve cap helps to protect the internal components from water and debris, thereby extending the life of the tire and maintaining optimal performance.

While the valve cap doesn't hold air pressure directly, that's a function of the valve and the tire itself. Similarly, it does not support the tire structure or maintain tire alignment, which are related to other components of the tire and wheel assembly.