A community before the advent of widespread literacy is dependent upon story and symbolism. In such a pre-literate world, the mind is subject to the mythical-- the sun is a force, a tree decides to sprout leaves in spring, the sea is a god, etc.
The skills of reading and writing provide a person and a community the tools to begin the journey of rationally considering the once mythical elements. More importantly, they allow thoughts to be analyzed and ideas to be processed. Finally, in a literate world, a person is no longer fully dependent on another for mediation between self and the divine. In the literate world, I can communicate with God and God with me.