It is professional

Your potential employer has just given you their time for the interview, not to mention the time spent reviewing your resume and coordinating the interview. It is good business etiquette to thank your potential employer and demonstrate that you are appreciative of the opportunity.

It keeps you relevant

You have managed to get your CV to the top of the pile, now make sure you keep it there. If an interviewer is seeing multiple candidates, you want to ensure that you stand out from the rest and writing an interview thank you email or letter helps you do just that.

It shows you are keen

Any potential employer wants a proactive employee – a follow up demonstrates your enthusiasm and commitment. In addition, you will remove any doubt about your interest in the role.