Roughness Remapping

I have been recently exploring physically-based rendering (PBR), but there is one part that still confuses me. I don't quite get roughness remapping, as described in the PBR section on Learn OpenGL. Specifically, scroll down until you find Geometry function. This remapping is described in several other papers / courses as well, such as Disney and Epic Game's PBR Course. Is this remapping solely artistic, or is there some physical reasoning in it? A quick answer and explanation would be quite appreciated :)

TL;DR What in the world is roughness remapping for?

• Ah, that makes sense. So, I'm guessing from your statement, that the remapped roughness is just to make the artists happy, right? It's not required. – Daniel Kareh Nov 13 '17 at 0:12
• Yes, it's unnecessary. If you look into Scientific Papers (like the original GGX Paper by Walter et. al), there is no remapping done. Also from a logical viewpoint, if you use $0.25$ as your roughness or $0.5^2$ as your roughness shouldn't make a difference. – Tare Nov 13 '17 at 7:18
• @Matthias: That looks like an interesting read. I'll make sure to read through it, thanks for the link. – Daniel Kareh Nov 14 '17 at 0:43
• Edit: I removed my comments to merge them in an answer. – Matthias Nov 14 '17 at 9:01

It is worth reading Moving Frostbite to Physically Based Rendering 3.0. These course notes explicitly use the terminology linear roughness and (non-linear) roughness, both in the text and code samples. Furthermore, it is also worth reading The Specular BRDF Reference which defines various BRDF components for the Cook-Torrance BRDF using the same non-linear roughness parameter $\alpha$ (defined as the square of the linear roughness $roughness$).