When you attach a picture to "displacement", and you set the placement of the image to the object, you get a relief effect on the material (Without changing the object).
It's connected to the material, but does not affect the parameters of the material (color, gloss, transparency, ...) which are themselves connected to the "surface" of Material Output. The two are independent. It's like putting a fabric on a warped surface, the form changes, but no the material.
I not know if I am very clear, without knowing the software, i may seem Chinese speaking ^^