This picture shows a single pin, two piece bracket with a line retainer . This setup would be used where a line, should it become detatched from the insulator or crossarm, may become dangerous should it fall below the crossarm. In such circumstances normal pole spacing and line tension usually keep lines up out of harms way . This retainer would most often used in rough terrain where one pole sits higher than other poles. Should the line become free from the higher pole there would be considerable drop and sag as there is no tension on the line. ( Shortest distance between two points is a straight line)