“What is the [survival] value of the feminine within men? The masculine within women?” asked author and Spirit Warrior  William Horden after reading  a recent post.  Here’s my best answer for now.
Consider these examples from Nature:  single-celled organisms and many plants and fungi reproduce all by themselves. Even some invertebrates and less advanced vertebrates such as amphibians and reptiles do not need a male parent. In the absence of males female turkeys can produce fertile eggs, and there are two known cases in which female sharks, raised in captivity without males, produced offspring genetically identical to their mothers.
The more advanced, multicellular and sentient forms of life, however, require two parents. Biologists think sexual reproduction may create more genetic diversity which helps organisms adapt to changing environments. Thus, life on Earth evolves from simple to complex, vulnerability to strength, self-preservation to species-preservation, and, most relevant to our question, unconscious to conscious. And the single most important factor influencing this direction is mutual cooperation between complementary pairs.
There is an inherent connection between the evolution of Nature and the evolution of Mind, or Spirit.  An ancient mystical saying says it this way, “As above, so below.” The original text reads, “That which is Below corresponds to that which is Above, and that which is Above corresponds to that which is Below, to accomplish the miracle of the One Thing.”  This means there is some unifying Plan, Intention, Mind, Source, Force, Ground of Being or One Thing whose laws govern both the metaphysical and physical universes. It also means the One thing needs both halves to accomplish its miracle. And what is the miracle?  Creating new, ever more complex and conscious life.
The Above half, Spirit (mind and consciousness), usually has been equated with “masculine” and the Below half, Matter (the body and the unconscious) with “feminine” which has led to damaging stereotypes. But these terms are not about gender roles or sexuality. They refer to the two modes of energy the creative drive takes everywhere, whether Above or Below. The ultimate reality is that if life is to continue to evolve in accordance with the Plan, or One Thing, there must be cooperation between masculinity and femininity at all levels: Above and Below, without and within, consciously and unconsciously, spiritually and psychologically.
Psychologically, we associate each half of the creative drive with specific qualities and locate them all in the Self.  As long as our egos are unaware or unaccepting of large portions of either half, we cannot fulfill our basic purpose or experience our greatest joy: to create.  But when we recognize that every quality we associate with masculinity and femininity indwells us, we break through the shell of dualistic thinking. The scales drop from our eyes, our creative potential is released, and we experience a blissful sense of oneness with the ultimate creative force: the One Thing. In alchemy, this enlightened state was known as the hieros gamos or Sacred Marriage.  (Next time I’ll share a dream about it.)
William concluded that, “… [the] sole purpose [of men having a feminine side and women having a masculine side] is individual well-being. It fulfills us to be able to incorporate our own opposite-complement.” Yes it does.  Why? Because this is how we unite the Above and Below and claim our divine inheritance.  Thanks for the inspiration for this post, William.  Thanks also to RamOsinghal for his reminders that all is divine.
You can find Healing the Sacred Divide here at Amazon.com and at Larson Publications, Inc.