After you interview, they will ask what you talked about and what kinds of questions you were asked. This allows them to keep track of how the company is interviewing, and to give the next candidate more information about what to expect.
This will mean that the recruiter doing so will have a potentially higher rate of success than a recruiter not doing so, since the candidate has this benefit.
Honestly, it's a pretty good thing. It's just good recruiters exploiting bad interviewers. It's better to design your interview around each candidate instead of recycling the same comp-sci questions that you heard in college over and over again.
Companies are really just testing how often people interview in most cases, but don't realize how bad the process is. They are using the same questions that every other company uses instead of creating their own, and it usually doesn't find good candidates as much as it finds people who memorize interview questions.
It made me wonder what this Carpenter would do if he was not aware of possible questions or he was given something he was not prepared for.