What's So Great About Gravity Falls?

Dipper and Mabel, twin prodigies, have always been surrounded by a strange man and his bizarre ways, but when they finally get to meet him, he's just some average Joe. He claims to be the owner of a "house of wonders" called the Mystery Shack, but Dipper's parents disapprove of this man's business. What's so special about Gravity Falls, Oregon? Is there more than meets the eye?