Reflectors can be used in many different locations for photography, and are used for adding and aiming light into photos. The reasons behind using a reflector are many as described:

The background is lit, and the subject is cast in shade. This situation would require the light of the background to be aimed back at the subject, with a reflector,  to illuminate him/her.

The subject is backlit; this refers to when the subject is directly positioned in front of the lighting. Using a reflector in this situation would bounce the light onto the face of the subject that would otherwise be cast in shade. While shooting in the path of direct light, a lens hood should be used to prevent glare across the shot.

Also, during an overcast day, light from above can cause dark circles under and around the eyes. A reflector can smoothen and brighten these areas to better the photograph.

 

There are also different types of reflectors; silver, white and gold. A silver reflector casts harder light onto the subject, which is good when wanting to create a harsh atmosphere/mood. Whereas the white reflector throws softer, more natural light on the subject, which is best for when wanting to create a more subtle effect in the photo. A gold reflector creates a warm, golden light onto the subject which resembles that of sunlight.

A reflector was used in my natural lighting post.