This enables polygons to make clearance around themsleves the same way as lines or arcs do. Speaking in technical terms, the new flag, PCB_FLAG_CLEARPOLYPOLY is the polygon alternative to the clear-line flag used on lines and arcs.

The real reason for adding this feature was to support arbitrary shaped smd pads with subcircuits - the smd pad needs to be able to clear other polygons. Being able to draw these thick islands in a ground plane is just a bonus.