Could someone help me to see the interviewers' point of view about why they are asking these questions?
From their point of view you showed up for an interview. That tells them that you are the kind of person who shows up for an interview. It says nothing about your motivations for doing so.
I have put a lot of work and effort into filling out the application form and preparing for the interview so obviously I am very interested in the role otherwise I wouldn't be applying for it!
I assure you this is by no means obvious to the interviewer. I have interviewed many many candidates, and a significant fraction of them show up with no interest whatsoever in the job. How, as the interviewer, am I to tell those candidates from the candidates like you, who are genuinely interested? Asking what you want to know is a highly effective technique.
You imply that it is obvious that anyone who would both apply for a job and show up for an interview wants the job. This is simply false, so stop believing that. I've had people show up for interviews who were practicing interviewing. They won't take the position; they'll just waste your time. I've had people show up for interviews who were intending on obtaining the given position as stepping stone to a completely different position. Those people are far more dangerous; they will suck resources out of your organization for no benefit in return. People interview for positions they don't want because they want to keep their options open, or because they want to find out secret information about the team and are hoping the interviewer will let something slip, or whatever. There are lots of reasons to take an interview other than "I want the job", and one of my tasks as an interviewer is to figure out if the candidate first off actually wants the job.
After I've determined that, knowing why they want the job, if they do, is a big part of knowing whether this person is a good fit for the organization and the position.
what is the best approach to answering them?
Please answer completely honestly; it will save us all time and money and help ensure that you get a position that suits you. The best answer to that question I ever got was "I don't want the job; the team I want to work for isn't hiring, your team is, HR suggested that I interview with you". Ten points to Gryffindor for honesty, NO HIRE, and now I know who in HR to stop trusting to have my best interests at heart. It was a win for everyone: the candidate wasn't hired into a position they didn't want, I didn't have an unmotivated coworker, and HR found out that they had an ineffectual recruiter who wasted everyone's time.
No, really, what's the best approach?
Beyond simple honesty, you tailor your answer the same way you tailor every answer in an interview: by considering the motives of the person doing the interviewing. Presumably they have a business problem and they believe that going to the enormous expense of interviewing and then hiring you, they might have a chance to solve that problem at a reasonable cost. (This is assuming that the interviewer is acting in good faith; see my comment above for thoughts on that.) If you want to make yourself seem attractive, make it clear that your motives are aligned with the business problem that you are solving. (This of course requires that you understand what business problem you are possibly solving in the first place; use the interview as an opportunity to clarify that if it's unclear.)
For example, if the position is sales at a car dealership, "I want to make a lot of money" is a perfectly acceptable answer. It shows that your goals and the goals of the business are totally in line. Commission-based sales generates profit for the dealership at the same time as it generates commissions for the seller. It's a terrible answer if you're applying for a job teaching math to high-school students. (Unfortunately.)