Why a hint of nervousness is useful shouldn't be that hard to understand.
It's a hugely important day, and what happens may determine what the rest of your life will look like.
Most people who come to the interview won't be able to fully hide that they are just a bit (or more than a bit) nervous about that.
If you are don't show at least a hint of nervousness, the interviewers will wonder what the reason for that is. The most charitable interpretation is that you're just able to hide it well - which has the unfortunate side-effect of also making you look somewhat insincere.
Less charitable interpretations, on the other hand, include things like not understanding how important it is, not taking the process seriously enough, being overconfident or simply having a sense of entitlement.
I'm not saying that any of those things are true for you, just that not showing the least bit of nervousness could make interviewers think that. In the worst case, a mean-spirited interviewer might simply see someone who seems confident as a good candidate for being taken down a notch.
Also, keep in mind that you're being interviewed about becoming a doctor, not being one - years of training and experience are part of what makes doctors comfortable even in the most awkward of situations. ("Oh, you have a commemorative statuette of the Eiffel Tower stuck up your rectum? Hmm, well, what we're going to do is the following...") You're not expected to have that skillset just yet.