Interviews have many advantages, the most significant of which is questioning people who cannot write their responses. This category also includes illiterate subjects or subjects who do not write as frequently as they speak. Oral responses from these individuals will contain much more information than would their written responses.
Another advantage of the interview method is that is actually results in a higher response rate than does the questionnaire. Many people who would ignore a questionnaire are willing to talk, with an interviewer who is obviously interested in what they have to say.
Another advantage of the interview technique is that you can plan to ask questions at several levels to get the most information from the subject. This approach is unique to the interview. The combination of structured and unstructured questions can provide depth and richness to the data and, at the same time elicit data that are comparable from one subject to the next.