The primary purpose of sending Fredo to Vegas was to give him R&R after his nervous breakdown following the Don's shooting. But the Don was forward-looking, and often had laminated motives for the things he did. As we saw, his reach extended to the movie unions in Hollywood, and he bankrolled Moe Green's hotel. So it's logical to assume that sending Fredo to Vegas was his opening move in expanding into the casino business, which Michael brought to fruition. The Don always profited from his moves. If Fredo succeeded as a hotel manager, so much the better. If he failed, the Don and Michael would know that, too, and know what they needed to do to overcome his weaknesses. I think that if Fredo had really been a success in Vegas, Michael might have remained in New York for a few more years. I think Fredo's failure accelerated his move to Nevada.