Thank you emails are a key piece of every interview process.
There is never a good reason not to send a thank you note after any interview. It doesn’t matter if you knocked it out of the park and they told you the job is yours already. (It’s never a done deal until the written offer is signed, sealed and delivered.) It doesn’t matter if it was only a phone interview. (Every interview matters.)
Why are thank you notes so important? It's all part of the interview follow up process. They give you one more chance to touch the hiring manager and make the case for hiring you. And they tell that hiring manager a lot about what kind of manners you have, what kind of communication skills you have, and if you are enthusiastic about this job. In many cases, it’s a tipping point. (See a sample thank you note for your job interview.)
One hiring manager called me a few days after a phone interview with one of my candidates. The manager felt that the candidate was qualified but was reluctant to say that he wanted to move the candidate forward. Then he asked if the candidate had talked with me after the interview and did the candidate ask for the hiring manager’s email address. Yes, I had talked with the candidate and no - she had not asked for the email address. That answer was the kiss of death for that candidate. That simple piece of information was the tipping point for him.
That particular job was in sales and marketing. Those managers often feel that if you don’t use all the tools to persuade them in your job search, you won’t use all of the tools you need to be successful in their position. If you’re in sales or marketing, not sending a thank you email gives them an excuse not to hire you.
In other jobs, the thank you note is a tipping point for other reasons. But it still can be the only thing separating you and another candidate in this competitive market. So use everything you can to influence them to hire you.