Purposive sampling targets a particular group of people. When the desired population for the study is rare or very difficult to locate and recruit for a study, purposive sampling may be the only option. For example, you are interested in studying cognitive processing speed of young adults who have suffered closed head brain injuries in automobile accidents. This would be a difficult population to find.

Your city has a well-established rehabilitation hospital and you contact the director to ask permission to recruit from this population. The major problem with purposive sampling is that the type of people who are available for study may be different from those in the population who can't be located and this might introduce a source of bias. For example, those available for study through the rehabilitation hospital may have more serious injuries requiring longer rehabilitation, and their families may have greater education and financial resources (which resulted in their choosing this hospital for care).