For when you're trying to focus on something but the iPhone keeps changing focus to something else (this doesn't happen a lot but sometimes the iPhone picks the wrong thing to focus on).
Additional, AE (autoexposure) lock allows you to meter the exposure on a single point of reference in the frame, and therefore will not change if you recompose from there.
This is very helpful when you are, for instance, taking a picture of somebody with a strong light source behind them (such as the sun) and their face is in shadow. Rather than trying to meter exposure for the whole scene, AE lock will allow you to select the exposure specifically for the persons face, ensuring your subject matter is properly exposed.
This may result in other areas of the photo being over-exposed (blown-out), however.
The iP5's HDR setting tries to mitigate this behavior by combing multiple images at varying exposures to try and keep both light and dark areas exposed properly.