For what purpose are visual signals used in addition to sound signals?

Visual signals are used in addition to sound signals primarily to enhance communication, especially in conditions where visibility is limited, such as fog. In such situations, relying solely on sound signals can be insufficient, as they may not provide clear information about a vessel's intentions or movements. Visual signals help to convey messages more effectively by providing a visual cue that can be recognized from a distance, allowing vessels to understand each other’s actions and intentions, thus reducing the risk of collisions.

While other choices mention aspects of maritime communication, such as distress signals or nighttime navigation, they do not address the specific need for clarity in low-visibility situations like fog. The combination of visual and sound signals ensures that communication is as effective as possible, particularly when visibility is compromised.