The simplest answer is that effective feedback is essential to the success of roleplaying. Research on learning going back decades tells us that learners, both in and outside of classrooms and seminars require feedback to improve. That feedback involves KCR - Knowledge of Correct Results.
In more simple terms, people need to know what they did well, what they need to change, and what they need to change to. Otherwise, very little learning occurs.
In roleplaying, feedback can occur from the person roleplaying, him or herself -- self generated feedback. Most often feedback is given by other members of the group. This is often problematic and is usually the weak link in standard role playing situations. Other learners are in positions to provide only certain kinds of feedback (for example, how they felt when the roleplayer said [whatever]. They are in less of a position to feedback on things that require expert knowledge or experience. Generally the quality of feedback from the group with respect to roleplays is also poor for reasons we will cover in our section on feedback.
The most common mistake trainers make using roleplaying is to rely on the group to provide feedback.