Standard front lighting can cause inconsistent and low contrast of a targets features due to shape, color and surface finish. Using a backlight ensures transmission of the light through the target into the camera. If the target is opaque a silhouette is generated creating a trong contrast and outline for stable inspection. Even if a target is transparent it may have a range of densities to it (such as layered material or liquid in a bottle). Contrast can still be achieved through the changes in the absorption and transmission of light in the target.
You can see an example of the use of this type of lighting and its comparison with standard light
An example of the use of Back Light can be found in the video below: