Handheld Light Meter Question

I've got a new EOS Digital Rebel and I have a question regarding the need for a handheld light meter. Other than convenience, what advantage is there for having a good meter as opposed to metering through the camera? Is the need for a light meter any different with digital as opposed to the old days of film? By asking these questions I realize I am exposing my ignorance and lack of experience. However, I do appreciate the chance to learn and the talent on this forum amazes me.

Mark

As somebody that is just getting into the portrait end of photography, there is a lot of guess work when not using a flash meter. Put it this way, you have a client wanting a portrait done, but he/she has to wait while you try a few shots to get your exposure and settings correct. You have a flash meter and you meter a flash and set your f-stop accordingly. Which would you rather do?
I am not using a flash meter either. I'm finding a real need for one now. I also would like know more about the uses of metering also.

I use a hand held meter( Sekonic L358 and L558) for two situations:
1. Flash Meter: Studio flash, to set ratios and overall camera settings.
2. 1 Degree Spot Meter or Incident meter when I can get right up to the subject: When the lighting is very uneven and I want to expose for a specific part of the image.
Regards,
Scott