Some research requires a sample that is not broadly representative of society but instead meets the purpose of the sample. For example, if you were researching domestic violence, a representative sample of broader society is likely to include many people who can tell you very little about the topic you are researching. Instead you would seek a sample that is precisely for that purpose, so access people who have had relevant experience or expertise. While it might be difficult to generalise from the findings, it is much more likely to produce relevant, informed and detailed data.