In essence, black recedes and white advances, or, hollows are dark and ridges are light.
The addition of a white line along the leading edge of a plane pushes space forward towards the viewer. A highlight naturally draws our attention and informs us of spatial depth.
The addition of white brings the lit areas of the plant into high relief allowing them to stand out more sharply against the background.
Artists such as Rembrandt understood that if they simply painted everything else darker, then the lights would automatically look brighter.