Oh but it is less valuable as we see here;<quoted text>
Most protestants adhere to sola scriptura. I guess you're an exception.
Why 'use' anything else? Because the gospel was preached orally for a decade or more before a single passage of the NT was written. The oral preaching of the Word of God by the same men who wrote the NT is no less valuable than the written Word of God. That's why.
3Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, 4so that you may know the certainty of the things you have been taught.
To be 'Certain' we go with what is Written.