I strongly recommend that you refer to John Creswell's book titled Research Design: Qualitative, Quantitative, and Mixed Methods Approaches, 4th Edition. The discussion and debate on the sampling method is tremendous and easy to understand.

Why do you need purposive sampling?

Using purposive or purposeful sampling method is often the only option researchers have when it comes to educational contexts simply because there is no way of performing a random sampling. As such, there is no intended bias in purposive sampling, which is sometimes called a judgmental sample. Due to a lack of random sampling, purposive sampling is sometimes open to selection bias and error. Even if you tried to eliminate selection bias , it can be difficult to defend your choices for participants. Consequently, great care must be exercised to avoid self selection and coverage errors. Notably, by using multistage strategy designs and triangulation techniques you can enhance the rigor and credibility of purposive sampling.