John 20:30-31 Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
1 John 5:13-15 I write these things to you who believe in the name of the Son of God that you may know that you have eternal life. And this is the confidence that we have toward him, that if we ask anything according to his will he hears us. And if we know that he hears us in whatever we ask, we know that we have the requests that we have asked of him.
And note that they were WRITTEN down.
The rise of many false teachings also prompted this need.
And as we can see from catholic "tradition" with mariology, the Mormon Book of Mormon and Doctrine and Covenants, the Koran, the written Word is still needed.
EVERYTHING that Jesus said to Satan in the wilderness contained...
...then it's good enough for me!