why do game textures that shouldn't be transparent have an alpha channel? observe

I have seen alpha layers in diffuse, normal, and specular maps. what purpose does it serve and is it useful in blender in any way?
This channel can be interpreted by the game engine as grayscale data and used for any purpose like a subsurface scattering map, which may be what is happening here.  This is called Channel Packing.
Reference: http://wiki.polycount.com/wiki/ChannelPacking