Moses reviewed the laws that the Jewish people had received from G-d at Mount Sinai. He began with the Ten Commandments.
The first account of the Ten Commandments -- in the Book of Exodus -- is the "real time" description of how this event occurred. The second account of the Ten Commandments is Moses' description of it, as part of his historical review of the Jewish people's Exodus from Egypt and their trek through the desert.
Reliving the first account of the Giving of the Torah allows us to experience G-d's presence in the Torah as we are studying it. This experience prevents us from forgetting that the study of the Torah is a spiritual encounter between G-d and us and not merely an intellectual pursuit. Hearing the second account of the Ten Commandments, couched as part of Moses' address to the people, enables us to employ our own, human intellect in the study of the Torah, in order to internalize it and absorb its message fully. In this way, the goal of making this world into a home for G-d is achieved.
From Kehot's Daily Wisdom