It's the little details that make a difference in the quality of your placements. Meeting candidates, asking the tough questions, actually checking references, and looking for respect from the candidate in the interview process are small things that lead to big dollars in placement.
One of those details for third party recruiters is walking candidates in the door for their first in-person interview. There are two primary reasons for this:
1) I want to control the situation: If the manager is late, I need to be there to talk to the candidate and find out when and where the manager will show. Same goes for the candidate. Some people don't follow travel directions well (I'm one of them), and in unfamiliar territory, it's best to have someone there in case you need to be guided towards the right building.
2) I like to prep the candidate beforehand. This involves a series of advice comments, followed by direct questions that are intended to get the candidate talking about something. Anything to warm them up.
One of the best questions I use is the simplest.
"What is the job you are here for?"
If you take the time out of your day to drive to a new company and meet with a manager, you should at least know what you are interviewing for. You should be prepared to answer this question, as simple as it sounds, because far too many people list the title of the job (often incorrectly), and then sit there with a not-too-bright look on their face.
If you are an SEO consultant - and you answer, "It's an SEO position," then you probably won't get the job. This answer signifies that either your recruiting firm failed to tell you specific details when narrowing down the applicant list, or you weren't paying attention and hoped to "wing it" in the interview. The question in the manager's mind at this point is why this person sitting in front of them is wasting their time when they don't even know what position they are interviewing for?
Take the time prepare yourself for the interview. Ask questions before you get in the door. Review them with your recruiter before you agree to meet the hiring manager. A bad interview doesn't just prevent you from getting one job. It often follows you around as managers talk to each other about this candidate who came in with a great resume, but messed up the interview in the first minute.