Two reasons:
1) Indispensable when shooting strobes (you can't measure the light from the strobes thru your camera ... of course you could play histogram games but it's not as reliable).
2) Most any other time when the light is fairly constant ESPECIALLY when shooting brides (bright white) and grooms (often dark suits). But in constantly changing light, I don't like to have to keep measuring, so I'll let the camera figure it out.
The reason?
Because your camera only sees a REFLECTION of light. So if you take a picture of a white wedding dress, the camera will try to turn the white into 18% gray, thus underexposing the shot. But if you take a picture of a dark suit, the camera will try to turn the dark color into 18% gray, thus overexposing the shot. And if you have both a bride and groom in the shot? Well, it depends on what you focused on at the time you pressed the shutter!
Your light meter, on the other hand, measures the actual light. You hold it up to where your subjects are and measure the light. Set your camera's exposure manually and then shoot with confidence, knowing that you are getting a good exposure.
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