John 17:17 Sanctify them in the truth; your word is truth.
If Jesus' own validation of Scripture is not enough for Catholics, then nothing is.
The word doesn't change, unlike *tradition*. It has been recoded for posterity and can always be referred back to when there is a dispute about what was said, unlike word of mouth or *oral tradition*.
The written word was VERY important to God. He commanded His words to be written down in many places. Matter of fact, HE wrote it Himself when it was really important.
The word of Jesus is truth whether it is written or not.