As a general rule, I'd say this question mostly serves a general politeness combined with a check to make sure you're still interested.
If an internal recruiter (or HR) calls you a few days later, 999 times out of a 1000 the decision would already have been made, and it doesn't matter all that much what you say (as long as you don't say anything particularly objectionable).
If a third-party recruiter calls you either immediately afterwards or a few days later, they generally can't influence the decision, so 999 times out of a 1000, it doesn't matter all that much what you say.
If an internal recruiter calls you immediately afterwards, they typically wouldn't have much of a say in the decision 99 times out of a 100, so it doesn't matter all that much what you say.
There is, however, a chance that if you have a good and acceptable explanation for something you did wrong during the interview (that may otherwise be the difference between getting and not getting the job), presenting that explanation may improve your chances of getting the job.
If the hiring manager/your future boss/etc. calls you immediately or after a few days, what you say could very well influence their decision.
In any case, whether it is or is not likely to change their decision, I'd suggest you:
- Express that you enjoyed the interview and learning about the role and company.
- Express excitement about the role and the company.
- Briefly highlight how you'd be a good fit.
- Maybe briefly touch on some things you could've done better (but don't focus on this too much, nor give too many details, unless they ask more detailed questions about that).
- Possibly treat this as an alternative to a "thank-you" / follow-up note, but unless you're speaking to someone from the company immediately after the interview, a separate follow-up note is very likely better.