The role of dichotomies in the first book of Genesis is closely tied to creation, and diversifies the Earth. Since this is the creation of heaven and Earth, and thus, of time and all beings in our history, there was no such thing as dichotomies to everything that was created. The opposites of light and darkness, day and night, land and sea, heaven and earth act as ways of separating the world, and give an idea of how it functions. Only God was aware of these differences, since He is an omniscient being.
Stemming from God, we get a sense that everything He makes will be in pairs of two, thus resulting in dichotomies. Everything has an opposite, and they fill the roles for which the other cannot. It is similar to the yin and yang philosophical thought. For example, day and night are opposites, but they will always be connected for the function of the world that God has made. If one is disrupted or ceases to exist, the world will go into turmoil.
Dichotomies show that something by itself is not truly complete. Everything can’t always be right, and at the same time, everything can’t always be wrong. Introducing opposites in Genesis I sets up the other books where it goes deeply into right and wrong.
I really like your view on dichotomies; how everything in this world comes with a pair, an opposite. “Opposites attract,” you hear it all the time but never really see how critical opposites our to our lives.