The primary benefit is in care gap reporting for providers from whom the ACO doesn't receive EMR data, with a secondary benefit in earlier case management identification when a patient receives care from a provider outside of the ACO's EMR integration. Having worked in claims data since 1999, I have had the (unfortunate at times) opportunity to learn that providers utterly hate being told that they need to do something they have already done, and far more so the longer that the reporting group "should" have known about it. Secondarily, it enhances their acceptance of reporting the more up to date the data is.
From another perspective: MSSP ACOs have the same uses for it as REACH ACOs.