Because when art directing the fresnel on, say, car paint - one needs to be able to have complete control over the incidence angle falloff. Although the default fresnel is a "mathematical" fresnel, our research shows that many car paints have a curve that is MUCH steeper and more weighted. By drawing a curve in profile it is much easier to get that photoreal look and tweak it to match reference.

I found a script that will tie the frensel to points in the graph editor to draw it there...but unfortunately it isn't supported by our renderer of choice

-John