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It's a scent marking--just like when they dive bomb their head onto the floor or swirm around on their back--it's either to leave their scent or pick up some other scent (stink roll)...
I discourage this in my dog (it's only my male that does it)
My pup does the face rub from time to time, especially right after a bath, or when he wears clothes (go figure?). I'm assuming this is to get the scent OFF. I also noticed he does the face rub when outside and he wants to get a scent ON - sometimes I notice him doing it on bird poop and promptly discourage it.