Shackles play an essential part in making sure leafs handle length changes correctly. Shackles are located between the spring and the hanger which is attached to the frame on one end (usually rear), to allow for movement, while the other end of the leaf spring (usually front) stays mounted to the chassis and cannot move.
Without a shackle's help, the vehicles leaf spring would not be able to flex and if you were to hit a bump the tire would lose contact with the surface of the road instead of being able to adapt and remain steady. There are many different features when dealing with replacement leaf spring shackles. Purchasing longer or shorter shackles can have it benefits as well, longer shackles allow for greater lift height as well as a substantial amount of ground clearance, whereas shorter will be a lowering shackle.