When I was interviewing for a new job sometime ago, a big style difference I noticed was that some companies interview candidates in pairs while others interview 1-on-1. Over my lifetime, I think about 1/3rd of companies interview in pairs (or more) while others do 1-on-1 interviews. As I was thinking about it, I think the pair interviewing is much better. Here are some of the reasons.
Have More People Meet the Candidate
Its at the interest of both the company and the candidate to meet more people. If the candidate only interviews with 3-4 people, it may be hard for the interviewee to get a gauge on the company and culture. If you double that to 6-8 people, they can get a better feeling. For the company, it may be best to do this to get more opinions on the candidate. Even if they aren't people the candidate will be working with, it's still good to get more opinions from people around the company.
For example, I know of a group that regularly sends their admin/secretary to interview candidates too. On atleast one occasion a candidate treated the admin/secretary with disrespect and blew off her questions. I know of a similar situation when a young engineer was treated disrespectfully when interviewing an older engineer. These probably weren't candidates you wanted to hire, so having an non-traditional team member interview the candidate ended up being a great idea.
Develop A Consistent Interview Style
There are many different styles in interviewing. Some like to ask one big design question, while some like to ask tons of tiny quick questions. Some like to jump in all the time in the discussion, while some like to just let the candidate do most of the talking. Some like to be more aggressive while interviewing (a griller), while some are more passive (let the questions come).
None of the above is "bad" or "good", it's just different styles. However, I think most would agree it's not "good" if all the interviewers are that divergent in style. At the minimum, a candidate might wonder, "What exactly is the culture/style of this place?" By pairing up the interviewers with other group members, a more consistent style can be developed over time. This is somewhat related to ...
Help Train Younger Interviewers
Following the above, it's also a good way to train younger team members on good interview techniques or style.
Develop A Consistent Interview Judgement
I once interviewed at a company that did 1-on-1 interviews and one person who interviewed me started <b>that week</b>. Now, I'm sure the interviewer was a good engineer and a good person. However, he had certainly not developed a rapport with the manager and team members yet. How much will you really value this person's opinion after the interview? If they say, "The candidate is awful!" or "The candidate is awesome!", how much will you believe them? It might be wise to pair up the interviewer with someone who has been with the company longer.
Give Consistent Information
As a follow up to the above, sometimes an interviewer doesn't know how to answer your questions about a certain topic. Especially when they are really really new. In a younger/startup kind of company, perhaps some of this is cannot be avoided. However, for a larger/more established firm, the company perhaps does not want this to occur. Interviewing in pairs can avoid some of this.