I have always had a pet theory (each time I have ever tried to explain it, it has been shot down rather quickly), but here goes:
WT was trying to establish their WT representatives as 'ministers' (for draft purposes) and, for those going overseas, as 'missionaries'. To qualify as ministers or missionaries they need to have credentials, and to have completed a course of study equivalent to mainstream schools. Many governments require minister credentials to include study of 'all religions'. A lot of Christian schools fail in this regard in that they only teach their own specific faith's beliefs. So the 'minister training' of JWs and JW missionaries needed something in the course of study about all religions. Voila, they produce a textbook for study that is held (at the time) in every KH library and is 'studied' in the 'minister' training. It mattered little that it wasn't really studied or looked at much; the fact that it was published and in the library was enough.