What is the main function of a camera's built-in flash?

The main function of a camera's built-in flash is to provide additional light in low-light situations. This is particularly useful when the ambient lighting is insufficient to properly illuminate the subject being photographed. The flash emits a quick burst of light, which helps to avoid underexposure and ensures that details and features of the subject are clearly visible. For example, in dimly lit settings such as indoor environments or during dusk, the flash can significantly improve the overall quality of the image.

The other functions suggested by the other options are not the primary roles of the flash. Enhancing color saturation is more about the camera settings and post-processing rather than the temporary light provided by a flash. Balancing exposure in bright sunlight usually relies on adjusting exposure settings, using filters, or seeking shade rather than using a flash. Focusing on moving subjects typically involves fast shutter speeds or continuous autofocus rather than any function of the flash. Therefore, the correct answer reflects the essential purpose of the flash in improving exposure in challenging lighting conditions.