In the Travels of Sir John Mandeville, the Blemmyes (along with many other "monstrous races") are described in detail as the foreign and exotic. Little value, if any, is placed on their actions or personalities but great emphasis is given to the face the sits large and prominent in the middle of the chests.
This physical construction brings up questions of anatomy, even indirectly, as there's no plausible way for the body of the Blemmyes to function. But for the sake of amusement--that is all Mandeville's tale really is, a source of freakish amusement--we're willing to suspend our knowledge of what's logical.
The strange appearance of faces where they shouldn't be (or the absence of a face altogether) has cropped up frequently in modern media. It shows up in movies, as in Harry Potter where the monstrous face of Voldemort appears on the back of Professor Quirrell's head, and in Pan's Labyrinth where the Pale Man's face is incomplete without his hands.
What is other is both terrifying and stimulating. It is what draws people to Mandeville's travelogue and what draws them into the thrills of horror movies, hard science fiction, and fantasy. And while we can more easily comprehend the multiple arms of a god-like creature or the wings of angels and demons, it is in the face that we anchor our understanding of human form.