Why is a job interview needed if I am the only suitable candidate for a research-assistant position?

There are a few possibilities.

It may be that an interview is required by the policy of the institution.

It is also likely that they want to get an idea of how you would fit in personally. If they are happy with your application materials and have no reason to doubt your honesty in that, they just might want to know if your personality seems compatible.

Yes, it is possible that you don't get the job. If they decide you are a complete jerk (sorry, nothing implied) they won't want to hire you and will most probably restart the search.

But, I'd advise that it isn't anything to worry over. Just be yourself and use the interview to help decide if you want to work with them. If you decide that they are complete jerks you will want to keep looking.

They may want to quiz you on aspects of your knowledge that are especially important to their work. Be honest in your answers, even if you aren't familiar. Be positive, of course, about your abilities and adaptability if that seems appropriate.

If you’re not a great candidate, chances are you won’t get the job.

I’ve sat on appointment committees for more than one U.K. Russell Group university. Pretty much every time the view of the committee is we’d rather make no appointment, than a poor appointment.

Last year, a committee I was on decided not to appoint to a professorial grade post, despite having a number of applicants who were already professors at other universities - they were just viewed as not good enough for the appointing institution.

So being a sole applicant far from guarantees getting the job. The university wants to interview you to discover if you’re a great candidate, or just good enough on paper. And if you’re in the second group they’d probably rather not appoint anyone and re-advertise at some point in the future.

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.

Good luck.