To negate the Normal map Banding artifacts during material authoring process. If I were to use the 8bit output normal map in a process that combines many many different layers of normal maps, the final output of such normal map will become significantly more “artifacted”.
This is an issue about “Perfection”, not an issue about “Good Enough”. To be honest, when more normal details are added onto the final 3D asset, those artifacts will be barely visible. But for example, if I model a lowpoly sports car, and have to use normal map to smooth out the surfaces, the un-dithered normal map(8 bit) will create banding artifacts, if I dither this 8 bit map, the final result will still have the banding effect due to the existence of the banding in the input image. If I have the 16 bit depth normal map(has no banding effect), and then dither it down into 8 bit, The banding artifacts will actually not being visible at all.(The noise still exist.)
This is the detailed demonstration of “why the 16 bit depth”.
In conclusion, for a final product, 8 bit depth normal map maybe good enough. But for material authoring pipeline(as a method in smoothing out mesh geometry/surfaces), 8 bit depth can not grant the best result. Or, in other word, is not capable of delivering the desired result.