Sending a thank you email is one of the highest leverage, lowest effort things you can do in a job search. Most candidates skip it, so the ones who send a thoughtful note stand out simply by showing up. It confirms your professionalism, gives you one more chance to reinforce a point you felt you under-explained, and keeps your name in front of the interviewer at exactly the moment they are deciding.
It will not rescue an interview that went badly, but for a genuinely close decision, it can tip things in your favor.