Russell was a businessman, as I understand it. If he was going to sell books, I'm not surprised he formed a corporation to do it. And he seemed to understand that running a religious group through a corporate entity provided certain protection from having it taken from him in the future. Rutherford certainly understood corporate scheming well enough, it's how he got control of the WTS after Russell's death.
And that is, after all, one of the primary benefits (if not THE primary benefit) of incorporating: it creates a legal entity that can be used to manage a business (or, to a degree, a religion), which can provide a level of continuity that doesn't exist with a person, who might suddenly be unable to manage it. I do find it interesting that Russell understood, from the start, the importance of having a tangible and legal way of keeping control of an organization that 'belonged to god.'