Many fringe inventors seek patents as validations. When they are often denied, the fringe inventors never seem to recover from the psychological blow, and spend the rest of their days fighting the evil patent office for supressing their invention that would save mankind. Of course, if they had such a great invention, they don't need a patent to develop and sell it. Duh.
On the otherhand, a patent to a bogus invention can be like a short term money printing press, since a lot of rubes are willing to part with their money to get in on the ground floor of a revolutionary new device that is backed by the full faith and credit of the US government patent office!
Well, you can't save everyone.