Nanite is just working around an inefficiency that occurs on small triangles that require screen space derivatives, which the hardware approxinates using finite differences between neighbors, e.g. for the texture footprint estimation in mipmapping. The rasterizer invokes additional shader instances around triangle borders to get the source values for these operations. That gets excessive when triangles are tiny. This is an edge case, but it becomes important when there is lots of tiny geometric details on screen.