The interview is valuable as a primary source of information.
I love it [interviewing]," he [David Bowie] said in 1999. "For me it's far preferable being on that end of it. I'm interested in how artists work. The process. How they got where they got, why they're like they are, how they do what they do. Those three things are what you want to find out about a person you admire. - David Bowie on his work for Modern Painter, from Uncut Magazine, January 8 2013
Interviewing is an excellent way to gather information that would not otherwise be available in written form.
An interview is a meeting between two or more people, in which a prepared interviewer asks questions and records the answers (by tape or through notes) of a specialist. The specialist's responses can include such things as technical information, statistical data, research, personal information, anecdotes, photographic materials, philosophical/political/social reflections, definitions, opinions*, and advice. *Keep in mind the difference between an expert's oral opinions, which are often given informally in an interview, and his/her carefully considered written opinions, which would appear in a professional journal or book.Cited from: City University of New York, Hunter College RWC, 'The Documented Essay Research Paper', Using the Interview as a Source.