To make a silhouette
I like silhouette images very much. They look bright and well represent objects with clear contours and interesting shape.
Silhouettes make the picture look more abstract. In case of shooting people, for ex. you can't identify who exactly is on the picture (and sometimes don't need a model release), but the meaning of the whole scene is absolutely clear.
Same happens when you shoot nature, still life and architecture (if it's not a famous landmark) silhouettes. They don't show a particular animal, plant or, let's say, a vase or church, but represent the whole class of such vases, churches, plants and animals.
To make a silhouette picture you need to have light behind the object. The explosure is set up by the lightest point which you still want to be clear and detailed. If in the end the silhouette is not dark enough Photoshop helps: putting down curves or increasing contrast make the contours clearer and remove useless details. Interesting background adds attractiveness to a picture.
Sometimes using colored filters makes sence.