sRGB is used for maximizing quality of a limited 8-bit image. It lifts the color values of the more important dark parts of the image before it gets saved. Because our eyes is more sensitive to differences in darker colors than brighter, the darker parts gets more room in the small 0-255 value space. When you open a sRGB file the values gets lowered again to look like they did before you saved the file.
The Roughness/Metallic/Normal/Specular is more for mathematical use inside the engine and not to be used to looked at directly like a Diffuse map. And you want to save evenly distributed values (linear) without the gamma shifting sRGB does. In linear the brighter values are as important as the dark ones. 
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