A coil spring is really a spiral torsion spring.

Automotive suspension coil springs generally have a seat that cradles the coils end as a way to distribute the load. At some point the cradle ends and a high load it placed on the coils wire. ( There are a few other coil end shapes but each can have a similar problem. ) Also, the cradled end is consider the " dead coil " since it does not contribute to carrying the load and the other coils are considered " active coils ".

Sometimes the coils open end will contact the next coil, this results in a notch being worn in an active coil causing a stress riser. Road salt can lay in the cradle causing the springs wire to rust, the weakens the spring and also causes a stress riser.

Broken springs should be replaced as I get concerned they might escape / move on a very bumpy road. A broken coil might slightly reduce ride height and slightly increase spring rate.