Why do you make a note of something or put a contract in writing? It might be to compensate for a faltering memory, or to recall an exact detail later on, or to hold someone to an agreement they made. Once it has been committed to paper, the words stay there as a testament to what happened or was spoken at a particular moment in time.
There is one underlying quality about things that are written down: They don't change.
"Don't Write That Down!"
The reality is that what we are not told is just as TRUE as what we are given in writing.
God's WORD on a matter is the decisive Truth on that matter. It is enduring because He is enduring. It is undiminishable by time because He is eternal. If He speaks it, there is no higher court of appeals.
It is written so that we can come back to God's character again and again and realize that what He has said is permanent and endures throughout all generations.