I know the main reason for a radio altimeter is for doing Cat II and Cat III aproaches. My question is: why do most twin turbo props that are unable to do a Cat II or Cat III aproach have a radio / radar altimeter? Is it just for a back up for putting in the wrong altimeter setting into the kollsman window... or maybe it's a required instrument for GPWS??? If it is not what I listed above how would you use the radio altimeter on a light twin turbo prop? thanks in advance.