With curves you are able to:

The idea behind Curves is all about re–mapping values. A pixel starts out at a certain brightness, and you change it to be brighter or darker.

Even if you’ve taken a good photo, chances are it can be improved or it needs to be adjusted to work in a collage or collection. Or even to just to intensify a mood. You can always make a good thing better – and curves is a one-stop-shop way to do that.