Maybe for when you land in Death Valley, CA where it's 250 feet BELOW Sea Level
The indication being discussed is based on height above terrain (as measured by radio altimeters), not height above sea level (barometric-based).
The radio altimeter readout is calibrated to indicate zero the moment the wheels touch down (with bogeys tilted, struts uncompressed and aircraft at average flare angle). On some aircraft, after touchdown, I've seen -10 feet displayed. Perhaps being able to select -20 might prevent activation after touchdown (so no flight crew distraction), even if your struts and/or tyres have deflated (in an abnormal situation)
Rgds.
NSEU