What is the purpose of using a base coat on nails?

Using a base coat on nails serves multiple important purposes, with one of its primary functions being to prevent staining. When colored nail polishes are applied directly to the natural nail, especially darker shades, the pigments can penetrate the nail surface and cause discoloration. A base coat creates a protective barrier between the nail and the colored polish, ensuring that the natural nail remains unblemished even after prolonged wear or frequent use of darker colors. This function is particularly important for maintaining healthy-looking nails over time.

The other options listed have their own advantages, such as adding shine or potentially speeding up drying, but they do not address the critical purpose of protecting the nail from staining. Strengthening nails can also be a feature of certain base coats, yet the primary and essential function in the context of this question is to act as a shield against discoloration.