Students are often unaware of the progress they are making, so communicating to them the areas where they are doing well or have improved is just as important as communicating to them the areas where they lack understanding or need further improvement. The positive feedback indicates which aspects of their knowledge should be maintained and built upon, where the negative feedback indicates which aspects should be adjusted.
Ambrose, S.A., Bridges, M.W., DiPietro, M., Lovett,M.C., & Norman, M.K. (2010). How learning works: Seven research-based principles for smart teaching. San Francisco, CA: Jossey-Bass.