The ability to plug in unique normal maps per BSDF node gives us flexibility as artists. For example, in rare situations you may want a diffuse BSDF to have a normal map while a glossy BSDF does not, like a clear-coat effect.
The best example to demonstrate this is “figured” wood. It’s a natural phenomenon occurring in wood grain that produces a 3-dimensional illusion. It looks like the grain is wavy but it’s actually perfectly smooth/flat to the touch. Watch the first 15 seconds or so of this video to see what I mean.
To create a figured wood material with Cycles, you would plug in a wavy normal map into the diffuse BSDF and then mix a glossy BSDF on top without that normal wavy map to achieve the figuring effect.
