This situation can come in many different forms, so it isn’t possible to give a blanket answer. That said, an example may help paint a useful picture.
Let’s say that you are down to two final candidates who are both strong options. They both check all the critical, required skill set and knowledge boxes. On top of that, the first candidate went to the same universities as several of their likely peers, has a similar professional background to other people who have been successful there in the past and their personality even reminds you of some of those co-workers. The other candidate checks the same important boxes but has a different type of background and personality that you don’t already see on the team. Perhaps they even have a personality that you worry may not blend perfectly with the rest of the existing group.
If you are looking for culture fit, you will consider the first candidate an obvious slam-dunk hire! In fact, if you do hire them, they will feel like the correct choice, and definitely the safe one — they’ll feel comfortable joining the team, be fully onboard quickly and do good work.
However, the culture add concept may tell you that the second candidate could actually be a wiser long-term option because of the unique insight and personality they will bring to the organization. It might be a bumpier experience than candidate one. New hires with a different personality than the rest of the group will go through growing pains and may have conflict with their teams more than the perfect culture fit would have. But purposefully selecting a highly qualified person who does things differently than the rest of your organization has the potential to be the best thing you ever do as a leader.
All of this, as has been mentioned, is under the assumption that the potential “culture add” candidate meets the minimum critical qualifications that you determined before the hiring process began. Everyone who gets to the final decision stage should have the skills and background needed to theoretically be successful in the role. Ideally, the decision maker’s job will be to select between multiple qualified candidates. In that case, use a culture add lens to help you think outside of your immediate, natural biases and give strong consideration to candidates who might feel like a slightly stranger fit than others.