The scriptures are testimonies and teachings of Jesus Christ. Prophets have been called in every age to teach the gospel to every man in their own tongue. While the code of conduct may change over time, and even the structure of the church may change over time, the plan of salvation will never change.
That's why we read the scriptures. We learn true doctrines and are taught by the Spirit regarding Christ's role in our lives. As far as knowing what we're supposed to be doing right now, we need a prophet.