The Torah has two levels: the secret and the simple one. First, there is the secret. It is for people who are like angels. Secrets are not for everybody, and they may be hard to understand. To them, Moses will "speak" - "medaber," which usually means harsh words.
The simple meaning is for the masses. To them, Moses addresses "words" - "amira", which usually denotes pleasant, endearing conversation.
But if, after all, the essential meaning is the secret one, why is the simple understanding needed at all? The answer is in the next phrase, "As the small rain upon the tender grass, as the showers upon the herb." When one is only beginning, they learn the simple meaning. They are like tender grass that cannot absorb and withstand heavy rain. After they have learned the basic facts, they become stronger and ready for the secret.
Art: L'Yerres, Pluie by Gustave Caillebotte