However, I am the only suitable candidate to this job, as I could see
by the official documents.
You probably mean that you are the only shortlisted candidate with suitable background or with shared research interests etc. But there are more than academic qualifications when recruiting a member for a lab. For example, nobody want to hire a jerk no matter how talented he is.
I once interviewed for a post-doc position in the UK. Part of the onsite interview was to talk with PhD students and other postdocs in the lab for an hour while the PI and co-PI interviewed other candidates. I thought they just wanted to make me busy, but that were a mistake. There were 2 candidates being interviewed via Skype and they had to do the same.
-Why would they want to interview me, if I am the only candidate, and they find me suitable to the job?
To evaluate you further, to get more information that is not available in the resume/ applications, to check if you fit with the culture of the lab, and so on and so on.
-What type of questions/discussion in the interview should I expect?
Only the interviewer can know.
It is a fact that I am not facing any competition, so what will the
purpose of this interview be?
Having no competition doesn't mean you will automatically get the job. It doesn't mean the interview will be easy either.
In the SF Bay Area (or Silicon Valley as poeple often call), there is almost no competition for software engineer, data scientist jobs. Often a company want to hire 300, and they can only find 30. So if you "pass the bar", you can surely get into Google, Fb, Amazon etc. That doesn't mean the interviews are easy, as the bar is very high.
-Is there any chance of not getting the job? I am really interested in the job, and I did not lie about anything on my CV. Can I still be
considered not adequate for the position?
There is always a chance for everything.
-Is the interview just a formality?
I guess not. You should prepare for it as much as possible.