This channel serves double-duty in the Alloy model as it is used not only for ambient occlusion, but also to dynamically generate a view-dependent specular occlusion term. This way, even purely specular materials such as metals can be correctly occluded, rather than simply darkened. Always remember that baked Occlusion/Cavity information should never be pre-multiplied into your base color maps.
Examples of tight vs. wide occlusion maps, general conceptual discussion of tight occlusion.
Use of occlusion map when alpha-ing out pixels in transparent materials