When your chevy springs get compressed(aka they flatten) they need to become longer.
Imagine it this way. You have a ruler stick(foot long) made out of metal. You hold it in your two hands(hands on the ends) and you push the ruler so that it forms a half circle. Notice that your hands pushed in to make it into a half circle? Well, when weight is put onto your springs, which flattens them out) they become a little bit longer. The shackles are what allows this.