The attached chapter related to collecting qualitative data should assist. It identified advantages and limitations - and does the same for focus groups as well.
I would have referenced Dean Whitehead but I see that he has already responded! I think there are a number of advantages to semi-structured interviews..particularly you get more of the "emic" or insider view if you let the interviewees "tell there stories" their way...they tend to focus on what is more tangible based on their experience rather than yours (as a researcher). I have attached a chapter by J. Smith that might also help (in addition to Dr. Whitehead's work).
The propositions/illustrations as per the following publications may further help, namely, the purpose of an interview serves to : gather descriptions of the life-world of the interviewee with the intention of interpreting the meaning of the described phenomena (see: Kvale, 1994, p. 149); gain a detailed account from an informant of the experience under study (see: Polkinghorne, 2005, p. 142); contribute to a body of knowledge that is conceptual and theoretical and is based on the meanings that life experiences hold for the interviewees (see: DiCicco-Bloom and Crabtree, 2006, p. 314); elicit the experiences, perceptions and feelings of the research participants (see: Edwards and Holland, 2013, p. 53).
Alshenqeeti, H. (2014) Interviewing as a Data Collection Method: A Critical Review, English Linguistics Research, 3, 1, pp. 39-45.
DiCicco-Bloom, B. and Crabtree, B. F. (2006) The qualitative research interview, Medical Education, 40, 4, pp. 314-321.
Edwards, R. and Holland, J. (2013) What is qualitative interviewing? London: Bloomsbury Academic.
Kvale, S. (1994) Ten standard objections to qualitative research interviews, Journal of phenomenological psychology, 25, 2, pp. 147-173.
Polkinghorne, D. E. (2005) Language and Meaning: Data Collection in Qualitative Research, Journal of Counseling Psychology, 52, 2, pp. 137-145.
Seidman, I. (2006) Interviewing as qualitative research: A guide for researchers in education and the social sciences. Third edn. New York, NY: Teachers college press.
Colleagues who have previously responded have given you very good answers. In-depth interviews could be characterized based on 3 aspects:
1) They are a qualitative research technique (the content of them is more interesting than the figures they could provide)
2) They allow you to enter in-depth in your topic of study. That is to say, due to the capacity of the interviewer to ask probing questions, it is possible to get to know in depth, aspects that are relevant for your research.
3) It is a very flexible technique. In this sense, they can be applied from a very open format where there are few guiding questions, to highly structured formats, with a very precise agenda of questions. Obviously semi-structured interviews are at a mid-point.