A camera, whether film or digital, can only record a limited range of subject brightness within any one shot. Sometimes it can be a useful composition tool to use this to our advantage and intentionally let elements in an image go black and become silhouettes. Silhouettes can make for powerful images, reducing our subject to a symbolic level and removing unnecessary details. To be effective, the silhouette must be an easily recognizable shape. A silhouette of a human form works particularly well, rendering it not as any particular person, but as a more universal statement.
Since silhouettes are rendered as completely black, be careful that they don't end up merging together. Also remember that what may not look like a silhouette to our eye could end up being rendered that way on film so merges can happen even when we don't see them beforehand. Use your camera's spot meter to help tell how objects will be rendered.