The most pronounced need for negative fill is when you're shooting a light subject against a light background. All that brightness in the high-key image can make a subject practically disappear into the background. But with a bit of negative fill (courtesy of black cards or flags) positioned on the sides of the subject and just out of frame, you'll create a dark edge to define the subject and separate it from the background.
Negative fill isn't just for high-key shots though. It's also a great way to add shape to a subject by enhancing the shadow side. Just as a white reflector lightens shadows, a dark negative fill card can deepen them further—which may be just the ticket to add drama to a low-key portrait, or to simply emphasize the shape of a subject in a tabletop still life.
Lest you think negative fill is only useful in studio, allow me to point out one of the easiest and most popular uses of negative fill in the natural world. Standing a portrait subject near to a tree, you'll find that the dark side of a tree trunk works wonderfully as a negative fill to add a bit of shape and definition to what otherwise might be a flat lighting situation.
Remember, it's not just about light, it's about shadows too. Without shadows, we'd have no shape, no texture, no drama. And with a bit of negative fill you can add edge-defining shadows, from subtle to strong.