Many people would benefit from a hand held light meter, to make that little difference in your photographs.
Your built in camera meter may not be extremely useful for situations such as taking a photo in a very light area, or a very dark area, such as may take place in high sunlight, or a darker room where your subject is wearing darker clothing. Photographs will come out with a great deal more detail in them if you're using a hand held light meter in a situation such as these.
The use of a hand-held light meter can help you immensely when the object in your view finder seems to be less well defined. If the contrast isn't the way you'd like to see it, or your photos are consistently less defined and clear than you'd like them, then most likely you need to invest in a light meter.