As for me it seems a little unusual, someone carrying their diplomas to the interview, if he/she isn't asked to. In my country (Georgia) usually, nobody brings his/her diplomas/certificates to the interview. I am not sure this is not country specific as mentioned in comments.
I used to go through many interviews as a potential employer and as an interviewer too. I've never brought my documents to the interview and I've never seen anyone doing such a thing. Usually, what interviewers do is asking some questions about process/goal/benefits of getting the certificate candidate mentioned in his/her CV. This questions partly ensure that candidate truly passed through these programs.
In other case, your certificates may be needed for your future career, because when it comes to promotion it's easier to choose right person if everything about employees is well documented. Also number of your certificates may grow and HR department needs to keep this information up to date, so you may be (and should be) asked to bring your new documents to HR regularly. In my last case, HR asked me to bring my documents after I was already hired.