You can use the exposure compensation feature to vary the exposure.
You can make colors richer.
For example, you may want to underexpose a sunset to make the oranges and reds darker.
Set the exposure compensation to a minus setting.
You can correct for light meter mistakes.
For example, if you photograph snow, or other light colored or toned subjects, in the sun, they'll be too dark.
You have to overexpose the snow using the exposure compensation feature.
Go to Light Meters Are Stupid.
Look in your instruction manual for exposure compensation.
When you use this feature, a number line will appear.
There'll be a black or green dot under the numbers.
Move a knob, or press on a rocker switch, to move the dot back-and-forth.
If you want to add light, overexpose, move the dot to the plus side of the number line.
If you want to subtract light, underexpose, move the dot to the minus side of the number line.
Go to Exposure Compensation.
Be sure to return the dot under the number line to the position under the 0.
You don't want to overexpose or underexpose every photograph.
To remind you that you're using exposure compensation, an icon of a +/- will appear in the viewfinder and on the LCD screen.
When you turn your camera on, always check to make sure the +/- icon is not present.