If an experimenter wants the conditions of the research to closely match those in the real world, a correlation design may be chosen. They were interested in the relationship between comfort level in a situation and sexual aggressiveness in men who were sexually aggressive. They found that higher levels of discomfort were associated with more inappropriate behaviors. It would be difficult to randomly assign a man to be sexually aggressive to investigate this relationship. So, to make sure the research generalizes to real sexual aggressives, real sexual aggressives were used as the participants.
Experiments can be difficult and expensive to conduct.
The corresponding correlation research might be simpler because participant populations are readily available. In those cases, researchers do correlation research to fully explore the relationship between two variables. Once the relationship is understood, it is then possible to do an experiment to determine causality. Later in the semester we'll look at some research on the relationship between judged control and depression that used this approach. You can pinpoint exactly what you want to know, and ask exactly that question.