Because horribly overexposed shots like this:
Are still usable, so long as you don't clip any important highlights. You still have the option to drag the exposure slider in Aperture or Lightroom down by three stops to end up with this:
In fact, this'll actually produce a cleaner image than if you expose it properly at the same ISO, since the highlights receive more bits in the RAWs than the shadows. Without RAW, the photo would have been stuck as is. Just make sure to turn on the "flashies" (highlight clipping) option in your camera. This'll blink black and white every place in the photo where you've shot it too bright to fix. As long as you don't see the flashies on anything important, you should be fine.
If you find yourself constantly darkening photos in post-processing, then you're doing it right.