The thank you letter after a job interview is considered a courtesy, and it's also an attention-getter. Opinions vary about the effectiveness of these letters, but it's generally agreed that the thank you letters are good etiquette. It's also a good way of following up on an interview, provided the letter is directed to the interview convener.
A further use of the thank you letter is to give additional information if required, or if during the course of the interview additional information was mentioned, but hadn't been previously provided, and you undertook to supply that information to the panel.
Example of Thank You Letter After Interview:
Job ref (As per references provided)
I'd like to thank you for your advice and help during my recent interview for the position of (insert full job title) which were very much appreciated. Please find enclosed as discussed at the interview evidence of my additional qualifications.https://www.sipa.columbia.edu/resources_services/career_services/current_students/career_resources/samples/Sample%20Thank%20You%20Letter%20for%20an%20Informational%20Interview.pdf