Yeah, I would too guess that reflect map is meant for not only older ways of doing it (non roughness based), but also variations in specularity. A typical place I would use this (although procedurally) is for shadow gaps in the material. Think of wooden floorboards with a gap between individual boards but is geometrically represented by a single plane.
I also tend to preview the map. If they contain little to no information in them, I just set the value instead. A 6k totally grey _REFL map doesn’t make any sense to me unless the end engine demands it. If target is Blender, it doesn’t require it.