Why did dinosaurs swallow stones?

Dinosaurs didn't "eat" rocks. However, a few plant eating dinosaurs may have swallowed them to help break down food in their stomach (similar to certain modern birds). These stones, called gastroliths, are very hard to identify, and thus it is unclear which dinosaurs actually used them. However, some possibilities include long necked herbivores (called sauropods), such as Apatosaurus and Diplodocus. Psittacosaurus, a small herbivore related to Protoceratops, was found with dozens of gastroliths in its stomach.