To put it simply, Yapp will help anyone and everyone to develop a mobile app. It's not meant to overpopulate the iTunes App Store nor the Android Market but rather to allow mobile users to essentially create interactive mini mobile sites for events, interests, etc. to share wtih family and friends.
"In the early going--Yapp is in private beta right now--it is focusing on apps built around events. So users can create new apps for things like weddings, conferences, birthday parties, and the like. And Yapp thinks that most people will be able to build their new tool in about ten minutes."