It's cheaper programming than TNA that gets about the same ratings with less exposure. Now that ROH is on Destination America it'll air in cities it hasn't before. ROH runs big shows in New York City and Chicago but never had TV there.
Hopefully this brings in ROH fans who decide to keep watching Impact after the episode of ROH has ended. This would mean Impact's ratings goes up. With increased ratings maybe Destination America will get the ad revue to justify not canceling Impact this September.