Chains are often associated with jumpstretch bands because they are used to increase the load during the concentric portion of the movement, as you reach stronger joint angles.
However the chains and bands differ in one very important way: the bands are actually trying to throw down the bar toward you (bench press) or throw you down (squat). So basically the bands provide an eccentric acceleration component that must be countered via muscle action.
The bands on the other hand are only added weight. The more chains are resting on the floor, the less weight is added to the bar and vice versa. However, contrary to the bands, chains don’t have that eccentric acceleration component. They simply allow you to increase the resistance during the movement.