Follow-up letter enhances positive and strong impression about your candidacy. Send it a few weeks after the initial application and you will leave behind other candidates with the same qualifications who have not sent it.

Your job search is not over after the interview. Send a thank-you letter addressed directly to the interviewer (HR manager, employer). It will prove your professionalism and consistency as well as show your continual interest in the job opening. Thank you letter also gives you an opportunity to mention the additional information you did not point out at the interview. (E.g. possibility to work overtime or fulfill extra responsibilities).