Being asked "why do you want this job" in an interview seems like a straightforward question. There is however, more to this question than may at first be apparent.
First we must ask ourselves why this question get s asked. Interviewers want to know that people they hire are suited to the job they have on offer and not leave after a short time for a nother job that more suits their interests or capabilities. Having to go through the hiring process a second time around - if a new hire leaves after a short time - is expensive, time consuming and disrupts other staff.
For younger people, perhaps looking for their first or second job, or those with less career experience the interviewer will be trying to discover how committed you are to the job they have on offer. There could be concerns you start the job as a temporary measure until a more suitable job is available or may just leave due to lack of interest in the job. The way to answer this question is to enthusiastically state your interest in the job and to describe 2 or 3 specific aspect of the job role and the organization to show the interviewer you are seriously interest ed and have done some research.
For people with a lot more career experience and success the interviewer will be aware you may have a broader range of choices in the type of work you do and therefore be looking for an answer that confirms your genuine interest in the role. Again your answer should be a statement of positive interest in the job and the organization and be supported by 2 or 3 specific aspects of it that interest you.
This is not the end of the matter though because interviewers often ask another question which appears to be unrelated to the first question but is in fact aimed at the same issue. You must be aware of the connection between these questions be cause it is designed to test consistency in your answers. The question is "what other jobs are you looking at?" If you answer this question by describing jobs that are totally different this will worry the interviewer that you are not committed to the type of job they have on offer. Answers to this question need to show that you are committed to getting a job of the "type" they have on offer, but that you are most keen on the job with them - for the reasons you already stated.
Employers like people they interview to be clear about why they want a job so when they hire them they will stay in the job and be a success.
What other potential answers and questions come up in interviews that appear straightforward but need careful answers?