This is how I’ve always understood them. If after you’ve had some time to digest how the interview went (and evaluate, based on the questions you (should have) asked during the interview, whether you think the position is a good fit for you) you still want the job, you send a quick email basically saying “Hey, thanks for meeting with me - it was nice to meet you / your team. Based on our interaction, it looks like this position would be a great fit for me / I’d be a great fit for it - here’s some things I took away from it (which also serves to show I was attentive / not just going through the motions) - looking forward to hearing from you to continue the process!” To your point, it’s not an ass-kissing email, the ‘thank you’ portion is just a polite formality to open the conversation.