Purposive Sampling method is used when the members of the population being studied are very difficult to find. For example, you want to study the impact of having narcolepsy (a sleep disorder that makes people fall asleep at inappropriate times) on one's work life. It might be very difficult to go around the general population and ask people if they had the condition. Instead, you would go to a sleep clinic where people suffering from the disorder go for treatment. You could contact a physician who specializes in the disorder, and randomly select from his list of patients.
The disadvantage of using this method is that your sample may be biased in some way. You could end up with a sample of people who had the financial resources to seek out treatment for their condition, or who were particularly attracted to the treatment methods of the physician you contacted.