c2263325_culty_Institute___What_is_PBL___WHY_
[WHAT:] Builds on/challenges prior learning [HOW?] If the case has some relevance to students, then they are required to call on what they already know or think they know. By focusing on their prior learning, students can test assumptions, prior learning strategies, and facts. [WHY?] 
The literature suggests that learning takes placewhen there is a conflict between prior learning and new information.
Problem-Based Learning Faculty Institute - What is PBL?
WHY?