It applies pressure lower down the nose then a cavesson and can help horses that like to fool with the bit or try to open their mouths to evade it.
It is no substitute for proper training, proper riding ( especially hands) and bit selection but sometimes it's an appropriate choice for an experienced horse that wants to get strong. Seen in the Jumpers, considered unconventional in Hunters where it advertises a potential issue to observers in a division stressing manners.
When opportunity knocks it's wearing overalls and looks like work.
It also stabilizes the bit (and lower jaw) without pulling the cheeks into the molars like a cavesson noseband can. Some horses seem to be more willing to relax their jaw when they feel it is supported, and aren't worried about biting their cheeks.