Deuteronomy 4:36 (ESV)

36 Out of heaven he let you hear his voice, that he might discipline you. And on earth he let you see his great fire, and you heard his words out of the midst of the fire.

Moses speaks of the events of Sinai from a different angle now, that what Israel heard and saw, namely, the voice and the fire, were in order to discipline them. God's display of his glory was done in order to make his people aware of their position in relation to him.