Canada

They're super important. They close the loop on power in the district, so that the superintendent is accountable to the community.

Trustees have power to choose/replace the superintendent, who is the "CEO" who theoretically controls everything, but really just Directors and Principals. Directors manage principals and vice-principals. They in turn manage teachers, who manage students (who complain to parents) and communicate learning achievement to parents.

Parents, in turn, can go to trustees if they have a complaint that is not addressed by the district and they usually respond well (if it's reasonable) since parents elect the trustees. (Trustees hardly get paid much, either. It's not a role to pursue for wealth.)