You are expected to do certain things: One of them is thank your interviewer with a letter or an email at least; Really? This is the first time I hear anything

You are expected to do certain things: One of them is thank your interviewer with a letter or an email at least; Really? This is the first time I hear anything like that. I can not imagine that an interviewer would want to get a mail or letter (who still uses these?) from every single interviewed person with basically no information in it. Am I wrong about this?