The best light meter will not necessarily replace the one in your camera, but it will teach you far more about light than your camera can. The best light meters can measure light more precisely than cameras, giving you a clearer picture of how the light is reflecting in your scene.
While the light meter on your camera is obviously useful, when you take a reading, it can only measure the light reflected by the subjects in the frame. This implies that an in-camera light meter will not be able to measure the quantity of light falling on your subject.
Using a portable light meter, on the other hand, allows you to wander around and check the light in different places of your scene to obtain a more precise image of what your exposure should be. Furthermore, most light meters have an adapter for measuring ‘incident’ light. While you probably wouldn’t use a handheld meter when shooting handheld, it’s an excellent utility when your camera is mounted on a tripod.
Studio flash photography is one of the areas of photography where a light meter may be quite useful. Most portable meters can measure both flash power and ambient light — something your camera cannot do. Some photographers choose to spend extra for a TTL flash setup instead, but this may soon add up. Instead, why not invest in a flash meter, understand how light behaves in various situations, simplify your photography setup, and be in complete control during the shoot?