Visual design attracts attention. Often through beauty. It might be the reason that they notice or engage with the interface.
Visual design communicates information to the viewer, such as how the interface can be used.
Visual design often expresses practical, intended things. An interface might be designed to make you feel happy, or to communicate brand identity.
Visual design often expresses the identity of the designer. This is related to personal style. This might not be intentional. Sometimes you can recognise a visual designer in their work.
A visual design exists for a user to interact with it. It provides elements that can be interacted with, like buttons.