But placing a gray card in front of an averaging meter IS about placing an average scene in front of an averaging meter. You would do this when the scene may be high-key or low-key because you do not want the unusual brightness distribution of the scene to affect your exposure.
Originally Posted by Stephen Benskin
Exposure using an averaging meter is about placing the average scene - with a statistically understood brightness distribution - and which has no reason to be in the middle - onto the characteristic curve of the film with the shadow down where it should be and the highlight up where it should be. All without metering the shadow or highlight individually. Just knowing where they usually are in an average picture.