Let’s say you're photographing an autumn landscape.
If you set the exposure for the fall foliage, say, f/8, the deep blue sky is washed out.
If you set the exposure for the sky, say, f/16, the fall foliage is too dark.
My students often suggest setting the exposure midway.
f/11 is midway between above exposures of f/8 and f/16.
Most often, averaging the two extreme exposures is a poor compromise.
In the example, neither the sky nor the trees would look as good as they could, if you averaged the two exposure extremes.
Instead, use the methods on the following sections to control contrast.