You should always send a thank you note after job interviews because it does the following:
- impresses employers with your follow through
- shows courtesy toward the interviewer
- shows your understanding of professional courtesy
- conveys your interest in the position
- provides you an opportunity to get back in front of the employer again, in case you have faded from the memory of an employer who met too many people too quickly
- allows you to introduce information that you neglected to mention in the interview
- demonstrates your skill at written (or emailed) communications
- meets your competition so you don’t lose an opportunity simply because your competitor sent a thank you
It is always smarter to write a thank you letter than to rely on first impressions.
Souce: https://www.job-hunt.org/job_interviews/job-interview-thank-you