Dr. Harlow's treatment of Phineas Gage, who survived a severe brain injury, helped advance the understanding of brain function and behavior. By observing Gage's changes in personality and behavior after the injury, Harlow contributed to the understanding of how specific brain regions affect emotions, decision-making, and social behavior. This case study provided valuable insights into the relationship between brain damage and behavior, leading to advancements in neuroscience and psychology.