Masks directly help prevent spread, the app does not. It's essentially a decision-support system, and I agree with Schneier that on an individual contact tracing-level, it's probably not a good one. But _perhaps_ it helps if it's well-designed and presents aggregates of probabilistic information well. If its underlying models are good it can, for example, inform a user where not to go because the infection risk is too high, after it has gathered data of frequent close encounters between otherwise "distanced" people in a location.