Do You Need An External Light Meter?
On a digital SLR (at least the Canon cameras I own) you have 4 metering options with relation to utilizing the light meter in the camera:
Evaluative metering (full auto)
Partial Metering (meters light in circle you see in the viewfinder)
Spot Metering (meters light on a certain spot in the viewfinder) - not all cameras have this and the options are dependent on the camera
Center Weighted Average Metering (the metering is taken at the center, then averaged throughout the entire scene).
Each should be used for the appropriate situation. In some higher end cameras, you can associate the focusing point with the light metering point - or even designate multiple metering points within the image to get the exposure right.
A light meter can help in situations where lighting is difficult and you want to get the perfect exposure in the scene when it is important to light the foreground and background of an image simultaneously or where you are in a studio lighting a composition that requires isolation such as when posing a model over a white background.
A camera's light meter measures reflected light. A light meter outside of the camera measures light falling on the subject.
Probably the best example is shooting in snow. A camera's light meter will give you a reading that makes snow gray (reflected light). An incident meter reading on the snow will give you a reading of light falling on the snow which is more white. That leaves you to determine which is more appropriate for the scene (I like gray snow during snowstorms because IMO it ads to the story - if I want to shoot a child sliding down a hill in snow - then I want to be sure the snow is white so the child stands out against it).
John Terrence Turner provides a great story of how while he was on an assignment at a Boeing Airplane Event how the use of his light meter beat out the images of all the other photographers and how he got the price he wanted for the use of an image as a result.
His article is at Stock Asylum
Here's a couple of tutorials that may help with further details....
How to shoot events