Why PBL? This is a great question! The Buck Institute for Education, the premier gurus for PBL worldwide, has some great resources to help outline that question in very simple terms.
ASCD has a lot to share about Why PBL
But my guess is that you may already be bought into the idea, since you're already here. Still, sometimes we run into folks who don't understand why we're working so hard to shift to PBL. When those educators and parents ask that tough question of "Why" be sure you go ahead and share some of these links.
If you'd like, maybe you have a few great posts, books, or articles at your finger tips. Please, do share them here in the comments for the rest of us.