I tried searching and there is no information about it. Blender manual says "to do" I did some experiment but still nothing is clear. Thank you.
My best guess is that it doesn't do anything now but later you'll be able to displace geometry with a Normal Maps rather than a Bump Map. Which means it will at least save us the hassle of converting a normal map to a bump map, but hopefully this means that we'll get better looking displacement since normal map carries shading information as well as height.