still confused with the difference between normal and displacement input

that’s normal input for BSDF node and Displacement for material output, visually it seems to me that by connecting normal map node directly to displacement of material ouput it gives the most desirable result, then what use is for Normal input of BSDF node?

all I can tell is Displacement here is pretty much the conventionall normal map, or am I missing anything here?

Right now, cycles displacement (changing the real position of the vertices on the surface) is still in experimental phase. What you have in displacement is just the normal (changing the apparent orientation of the surface), exactly as if you’re feeding the bump node into the shader. The only difference is that the normal will appear in all the shaders, without any control.