Why precedence is needed
A mapped stream already represents a hydraulic drainage condition. DEM-derived surface drainage at the same location often represents the same physical drainage signal. Applying both simultaneously would double count river drainage.
Away from rivers
Outside mapped streams, DEM-derived drainage remains valuable. Rivers may be wider than mapped lines, wetlands may be missing, and shallow groundwater discharge may occur outside mapped hydrography. DEM drainage lets these features emerge from the solution.