Every one of my son’s interviewers volunteered and took time away from their jobs or private lives to talk to my son, answer questions, and then inform the schools of the interview. I made my son send an email the day of the interview letting them know he did appreciate that they did so. Imho this just isn’t optional.
I have heard that schools will print the thank you email or put a copy of it (if forwarded by the interviewer) into the student’s admission file.