Concealers come in different shades to cater everyone in the market. It is most helpful to those who have dark spots on their faces as it can be applied to cover certain blemishes. People with eye bags surely find this cosmetic helpful since it can also make you look like you’ve never had lack of sleep in your life.
Since a lot of shades are available in the market, keep in mind though that you should buy something that’s a notch lighter (and only a notch lighter! You wouldn’t want to look like you have “owl eyes” don’t you?) than your foundation.
Photo taken from http://www.bbc.co.uk news stub.