Thank You notes have been part of the job search tradition for many years, and although the typical delivery format has been updated (email is fine), I tend to find that candidates still primarily view the note as a gesture of gratitude or a formality. I suggest that job seekers instead view the Thank You note as an opportunity to maximize their chances of an offer.
Sending a Thank You note does allow a candidate to check off the "Meets Minimal Social Obligations" box that an employer may wish for in potential employees. We will lead the note with an expression of appreciation, but an effective Thank You note will go farther than just a show of gratitude for an hour of someone's time.
What other mileage can we get out of the note?
Perhaps the most important potential use of the Thank You note is as a reminder to the interviewer. This is your "Don't You Forget About Me" moment. An interviewer might meet with a handful of candidates in a day and as many as 20 in a week during active hiring binges, which can make it more challenging to stand out from the pack (particularly if your interview falls somewhere in the middle of those 20).
Once we've shown the obligatory gratitude, we want to somewhat tacitly remind them of both who we are and what we want them to remember about our interview. As opposed to writing "In case you forgot, I was the chubby guy in the Uber t-shirt," we want to refresh the interviewer's memory by conjuring up a part of the interview that seemed to go particularly well. Good examples could be a project that had specific relevance to the employer, an interesting solution to an interview exercise, or even some shared professionally painful experience broached during the conversation. Think about what was memorable and make a comment about that moment, and hopefully the interviewer will have felt the same way.
Follow Through and Continuation
The Thank You is also a chance to provide a bit more information that might bolster your candidacy or demonstrate that you've given more thought to the interview since it ended. Maybe you mentioned a language/tool you've used that the interviewer was unfamiliar with. Sending a link with further reading material related to a discussed topic could show thoughtfulness, prove that you are someone who follows through, and can also start a continued dialogue with the interviewer that can serve as a type of bonding.
Lastly, the Thank You is a way to summarize your interest in a more eloquent manner. Interviews often end rather quickly and abruptly, and even the most skilled candidates may not find an opportune time to express their interest clearly and effectively. The close of the Thank You can relay both your level of interest and the reasons.
Instead of just viewing a Thank You note as an obligation, use it as a tool to highlight and revisit the best parts of your interview.