This is an easy question to answer. The councils were to affirm, not invent, doctrine.
One example of these doctrinal councils happened at Nicea in 325 A.D. The various church representatives came together to discuss the deity of Christ. Using the scriptures, they showed that Jesus was both human and divine. Some heretics had claimed that Christ couldn't be human while others were insisting he couldn't be divine.
These delegates also affirmed the twenty-seven canonical books.wich we have in the New Testament. The importance of this is that we can depend on those books to be the product of the Holy Spirit and not of mere human invention.
Look at what 2 Peter 1:20-21 (KJV) says. "Knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost."
Not only prophecy but the entire Bible is from God Almighty. Paul wrote in 2 Timothy 3:16 (BBE) that, "Every holy Writing which comes from God is of profit for teaching, for training, for guiding, for education in righteousness:" Some translations use "God-breathed" to describe the origin of the scriptures.
Furthermore, the scriptures are provided to strengthen our trust in Christ. Look at what 1 John 5:13 (KJV) says. "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God."
We can trust that these words came from God through various authors by the power of the Holy Spirit. Acts 1:3 (KJV) reads, "To whom also he showed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God:"
I'll be writing more about the infallibility of Holy Scripture in my next book called You Think You're Going to Heaven? Eternity is thee most important matter we must ponder. Neither do we have an opportunity to change destinations after death so it's vital we get it right while we're here.