Moses had said to the people, "If only you would pay attention to the voice of the Lord, your God." In those days, God's voice was heard especially through the voices of prophets. So if there were no prophets around, where would people go to "pay attention" to God's voice? Up into the sky, or across the sea? One alternative was to recall what previous prophets had said. This became the oral tradition.
In later days, God's word took written form. Most of the people were illiterate, so somebody would have to read God's word to them. Even if some people could read, maybe they developed visual limitations that made it hard for them to read.
Nowadays, people know how to read, but some just don't care to read any books, including the Bible. Should anyone be wondering what God has to say to them, Moses' advice applies to anybody who is wanting to hear what God has to say: "It is something very near to you, already in your mouths and in your hearts." We're challenged to become attuned to God's voice within our hearts. Once we've caught on to what God is saying, Moses concludes: "Now you have only to live it out."