Base reflectance is not the same as fresnel reflections. These are reflections that occur when the light hits the material at grazing angles, and the beauty of this node is that it handles these automatically. No need to even think about them. In this scene setup the fresnel effect can be seen on the side of the cylinder as the light here is coming from behind the prop and hitting it at a sharp angle. The smaller highlight at the top centre of the cylinder is caused by the second area light in front of the prop, and therefore shows base reflectance.

This brings up another important property - Roughness. The physics are clear that as the angle of light hitting a surface approaches 90 degrees the reflectivity of that surface will approach 100%. This means that all surfaces behave like mirrors when the light hits them at grazing angles. But while the science might state this, it’s not what I see in the real world - a dead leaf never appears mirror-like no matter what angle I turn it to the light. Why not?

The answer is that the surface of the leaf is rough at a microscopic level. If you could see it in enough detail it would look like a mountain range, and the light gets caught, trapped and bounced around deep in the valleys. It cannot escape easily, or by the same route it arrived. Mirrors are smoother, and the smoother the surface the lower the chances of the light being trapped, and the easier it becomes for it to escape and bounce back in a straight line.

Therefore roughness is extremely important when defining a material. If the geometry defines the primary shape of an object, and the visible surface relief is defined by bump/normal/displacement, then the roughness defines the invisible surface relief, which has a huge effect on how the light is treated when it hits it.

The amount of light that falls on a surface does not change, so the amount of light reflected back from it does not change, but the greater the roughness, the more diffused and dull the appearance of that selected light.

Here the setup is the same but Roughness has been changed from 0.5 to 0.05 -

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