Heavy ion collisions can be central or peripheral and it is crucial to distinguish between them. In peripheral collisions, only few protons and neutrons are involved and most of the nucleons fly along the beam line without participating in the collision.
The ZDC measure the energy carried away by the non-interacting nucleons (“spectators”), that is the measurable quantity most directly related with the centrality of the collision. It is therefore used as centrality estimator in order to distinguish between central or peripheral collisions. In heavy ion interactions ZDC is also used as luminosity detector.