What is the significance of the Greisen-Zatsepin-Kuzmin limit in the study of ultra-high-energy cosmic rays?

The Greisen-Zatsepin-Kuzmin (GZK) limit is important in the study of ultra-high-energy cosmic rays because it sets a theoretical upper limit on the energy that cosmic rays can have due to interactions with cosmic microwave background radiation. This limit helps researchers understand the origins and propagation of these high-energy particles in the universe.