To say that Scripture is necessary is simply to say that we need it.
We need this in two ways.
Firstly, we need Scripture in order to be saved, meaning we need Scripture because God’s general revelation to mankind in creation is inadequate to give mankind knowledge of salvation. The only place mankind learns of redemption is in Scripture, and so if anyone is to be saved they must have the Word of God. When Jesus was being tempted by Satan in the wilderness He responded to Satan’s temptation in regard to His hunger by saying, ‘Man does not live on bread alone, but by every word that comes from the mouth of God.’ (Matthew 4:4) Romans 10:17 also shows this saying, ‘Faith comes from hearing, and hearing through the word of Christ.’
Secondly, we need Scripture in order to live Christian lives, meaning we need Scripture because only there do we find a full explanation of who God is and what He demands of us. Just as the only place mankind can learn of redemption is in the Bible, so too, the only place mankind can learn of obedient Christian living is in the Bible. This is why Israel received the Law; that they would know how to walk as please God both in the wilderness and in the promise land. This is also why we, the Church, now have the Old and New Testaments; that we would know how to do life in this world so as to glorify God in all we do, walking in a manner fully pleasing to Him.
We need Scripture, it wisdom from above that transcends human wisdom. No Scripture, no salvation, and no salvation, no Christian living.