The purpose of this parameter is to simulate wet surfaces due to rain, or covered in liquid like an oiled or soap solution (see wetness oiliness to better understand this).
This is a very subtle effect and is more appreciable in dark (preferable rough) surfaces. Also, it is more appreciable on close-ups.
Controls whether or not the surface will look wet (transparent liquid over the surface) or as your base shader.
From left to right, values were: 0.000 | 0.500 | 1.000. The rest of the parameters were left as default.
Min = 0 | means no wetness, making the surface to look as dry as it supposedly was previously
Max = 1 | means full wetness
Interpolation | between 0-1 is a linear opacity transition
Default = 0
Controls the mix between the color set in the “Wetness Color” slot and the one that we defined as Oiliness. This Oiliness color can not be altered, but just mixed.