Time or budget is limited: Classically, qualitative research would precede quantitative measurement. However, adding a discrete qualitative stage to a research design can add considerable time and expense to a study. A single, semi-structured stage is can often be a helpful compromise.
The universe of target respondents is limited: This is especially observed in b2b markets. In these cases, each interview must work hard in delivering a range of information about the target audience. Typically this includes both deeper understanding and measures of key issues.