What would he have to gain?I don't know. I could speculate, but Joseph never gave a confession or anything approaching one, so far as we know. There's one affidavit from a neighbor where he purportedly says that the Book of Mormon thing was a rouse done for fun. But who knows? It could have been for attention. To drum up treasure-seeking business. For profit. (He first attempted to sell the copyright in Toronto.) To bilk Martin Harris. For power. Women. Self-protection. And on and on. There's no shortage of reasons why people do things. What's unique about Joseph was that he actually accomplished so much.
My best guess? All of the above. The motives for his actions changed over time, as do yours and mine.