That specific detail is for on external periphery plinth beams.
Many times it happens, your plinth beams bottoms are way above the ground level (mostly if you look at bunglow constructions). efforts taken to backfill and finishing works on ground level may results into settlement due to spillage of fill from below plinth beam bottom area.
to prevent spillage of fill and to ensure there are no settlement issues, this fin is provided as an additional safety feature to ensure plinth fill and compaction happened correctly.