The arguments in the previous post, and probably many more, give enough reason to suggest that even if we did not have a notion of canon and did not regard anything beyond the OT as scripture we would still find the NT writings to be trustworthy and worth reading. We do not need a doctrine of scripture to make the historical narratives of Jesus and the early church absolutely essential reading. Similarly, the teachings of the apostles is also essential (and trustworthy) even if we haven’t included the documents in which they are recorded into a formalised canon of scripture.
However, The doctrine of scripture and of canon gives us more than this. It allows us to equate directly the words of Paul and the other NT writers with the words of God himself. A doctrine of scripture gives us assurance that the NT writers have not made some mistake on a vital point. So, while the writings of Paul, say, would carry much authority because of Paul’s importance as an early believer and a person who saw the risen Jesus, they carry much more authority when they are identified with the very words of God.
A doctrine of a closed canon allows us to be sure that we have not missed any important documents and that we need not continue to add documents to the canon. It allows us to assert that scripture is sufficient as we have it and that God is not going to reveal anything substantial that he has not revealed already.