It's not functioning as a thank you, it's honestly just an indication that you're willing to take an extra 5 minutes to do something when the stakes are high.

If you really want the job, how you treat that conversation might be similar to how you treat a client once you're hired. If you don't really want the job, or you really don't want to send a thank you, that's fine, but with 5 applicants to choose from, wouldn't they opt for the candidate who put in the best/most work?

I'm not saying it isn't annoying and transactional and a lot to ask, but as someone who's applied for hundreds of jobs, the reason to do it still feels clear.