Professors have many responsibilities - to their students and postdocs, to their departments and colleagues, to their university, to the scientific community, and to the public. When on a doctoral committee, for example, a professor's duty is to make sure that the candidate's thesis is rigorous and careful, and that the student actually knows what they're talking about. Obviously primary responsibility for supervision of the student lies with the advisor(s), but the committee members are not window dressing; they're supposed to serve a valuable role in upholding the quality of the work.
I have a colleague at another institution (names and circumstances have been changed here; I'll say no more about specifics) who really had to put his foot down several years ago, as a committee member, to make sure that a student (the last one of a just-retired professor) didn't hand in a thesis sufficiently fringe that it bordered on pseudoscience. It was pretty clear that the advisor would have been willing to let this slide (!) for the sake of getting the last student out the door. My colleague (junior faculty at the time) had to push hard to make sure that this got resolved. Eventually the student did complete an acceptable thesis (on a much more mainstream topic) and got the degree. This colleague just recently came across the former student again, and was disappointed and sad to see that the fringe aspects of science are back in what he's doing. My colleague is now feeling (irrational) guilt about this (that the former student is now credentialed and pushing this stuff), even though the actual thesis was fine in the end. This does raise the question, though: how much of a gatekeeper should a committee member be?