Its useful if you wish to save the obj as a morph target. For example, you adjust a face the way you want with 20 morph targets and wish to save that out as a new morph target. select the head only when exporting then turn off all other check boxes except for Save as Morph target. The obj can now be loaded as an MT for others to use. It basically stores off the obj like bloodsong says without UVs and without a world transform. If you want to export an MT you can also use Morph manager which does all this for you.