An interview scenario demands a level of preparation by both parties, and in truth, the face to face interaction only builds upon foundations established either by the CV (for the interviewers) or own research (for the candidate).
Job descriptions provide a level of information on the role applied for, though these can often be technical in nature rather than contextualising aspects like the employees, work environment, clients etc.
As part of the recruitment process, a good recruiter will offer as much guidance and support as possible; including what they know of the company and the position. However, parroting a recruiters words back to the employer displays a lack of initiative; and simply repeating things heard does not mean the candidate actually comprehends them. Candidates conducting research themselves not only gain a deeper and broader insight into the role but also have the benefit of interpreting it themselves. Found something out about a firm you're not sure about, or don't understand? Ask at the interview!
Consummate knowledge of a company is not a guarantee of a job offer, of course. There is a multitude of factors to consider, which vary depending upon the nature of the role. Articulation, presentation, enthusiasm etc. are all factors that cannot be assessed accurately through written communication, hence the importance of the face to face interview. However, even those things are dependent on preparation; after all, how can enthusiasm be genuine if you don’t know what you're being enthusiastic about?
When the interviewer asks why you would want to join their company they will expect and answer specific to them and will spot generic drivel!
The key to a great job interview is preparation. As somebody who has interviewed thousands of candidates I can tell you that it is very sad to meet a candidate who is smart and friendly -- but also, sadly unprepared for the interview. When you ask a candidate, "What do you know about our company so far?" and their answer is, "Not too much, to be honest -- just what's in the job ad!" it's a sad moment.