Typically the chain acts as a variable counter weight. As the span descends and the counter weight rises there is, without the link chain, less effective counter weight mass as the cable shortens on the counter weight side and the load increases on the span side as the cable lengthens. The span is very carefully balanced against the weights, one at each end. As the weight rises it lifts a compensating weight in the link chain to offset this change in load. This system allows for smaller motors and less wear on the brakes and the system always being in equilibrium.