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.
This parameter lets you decide if you want to use as a last layer of effects the "Dust" section or the "Wetness" one.
<aside> 📌 Important This parameter works as a "toggle". Values ranging from 0 to 0.5 will show all the previous + the "Dust" one (and hide the "Wetness" layer), whereas values greater than 0.5 will do the opposite. So it is recommended to be used as 0 (when you need "Dust") or 1 (when you need "Wetness") values to avoid confusion. Also, keep in mind that you need to enable (put a value greater than 0) on the main effect sliders in order to make the effect appear.
Default = 0
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