Why do/did Fundamentalist Mormans believe that plural marriage is essential for exultation?

I apologise if I've misused the term "Mormon," but I wasn't sure what else to use. What is the reasoning or theory behind the "The Principle"? Why would it be considered superior to monogamous marriage? Was/is it politically motivated (in that it would help grow the faith with more babies) or is there an actual religious / theological reason?