I am assuming it's just asthetics to further differentiate the differences between Air and Pro which the prior generation looked incredibly similar in fact without seeing the names it was difficult to tell which was a Pro and an Air based on the keyboard (tho the Touch Bar) was the give away.
IMO I prefer the look of the colour behind the keyboard to have it stand out to type easily than the darker look of the Pro.
But just my guess. All we know its some sort of Thermal protection.