Sometime, even the most sophisticated DSLRs can make a complete mess of the your exposure, with the result that your subjects will be too light or too dark. Think about some landscape photos you may have seen where the sky looks great but foreground objects are too dark, snow photos where the snow looks grey, or people playing on a beach in bright sunshine, but it looks like they were playing at tea time because the images were dark.
This happens because camera metering systems attempt to average out the exposure to give a mid grey tone, (not grey in colour!), this usually works well because many images have both bright and dark areas in the scene. When the tones of these areas are averaged out, they quite often end up as mid-grey, but when a scene has very prominent bright or very prominent dark areas, this averaging out method can cause under or over exposure.
The lovely bride Aimee above, has beautiful light skin, but because the wall and her dress are quite dark toned (and her face and arms are quite small in the frame), her skin would have been over-exposed if I hadn't used -1.3ev exposure compensation.
In these two short video clips you'll see how the auto metering system of the video camera is fooled when zoomed in.