The publicly reported trading volume at Mt. Gox included the fraudulent transactions, thereby signaling to the market that heavy trading activity was taking place. Indeed, the paper later shows that even if the fraudulent activity is set aside, average trading volume on all major exchanges trading bitcoins and USD was much higher on days the bots were active. The associated increase in “non-bot” trading was, of course, profitable for Mt. Gox, since it collected transaction fees.
But the Willy Bot likely served another purpose as well. A theory, initially espoused in a Reddit post shortly after Mt. Gox’s collapse (Anonymous, 2014b), is that hackers stole a huge number (approximately 650,000) of bitcoins from Mt. Gox in June 2011 and that the exchange owner Mark Karpales took extraordinary steps to cover up the loss for several years.