This is a great feature. It is needed when shooting with any of the "M" type lenses to compensate for exposure differences with the older manual lenses. It is just another reason for getting a K20D instead of the K10D bargains out there.
Dave
Originally Posted by blende8 
Very good!
This is useful when you use the Green Button for getting the correct exposure.
Some lenses, and some split-image screens, need a constant correction.