The first program committee on which I served was SODA in the last 1990s. It was eye-opening for me. Never before had I heard frank discussions about what made some research questions "good" and others not so good. Never before had I heard in-depth discussions on what made a result worthwhile. It was a great education.
Previously, it was always a mystery to me how one would pick a problem to work on, and my choices were impulsive and not reasoned out. My criteria used to be: "Is it fun? Do I feel like working on it? And [after working on it a while:] Am I getting anywhere?" Afterwards, I could assess possible research directions in a much more systematic way.
In addition I also learned that many of those questions are discussed by mulling over the introduction of the submissions, and that helped me organize my writing and figure out what to put in the introductions of my subsequent papers.
For people who went to graduate school at a place where there is a tradition of forming PhD students in those matters, being on a program committee might not be such a big deal, but for me, it was a great experience that affected my research methodology.