They keep snow, dirt, and road salt out of your ice house, and stay in place when traveling over the roughest roads. They also keep out rodents in the summertime. The handle folds flush, is easy to grab with gloved hands, and won't twist out.
ORIGINAL CATCH COVER SQUARE HOLE COVER
Covers are most often used to retrofit ice houses that have square holes in the floor already, or can be used if you prefer to cut your holes in with a circle saw.