What Is A Lutheran?
These two things--believing in Jesus alone for forgiveness, and trusting all the Bible to be the absolute truth and source of what we should believe and do--goes a long way toward explaining what it means to be a Lutheran. We never want to get away from these basics.
This "book" (also called "The Lutheran Confessions" or "The Symbolical Books of the Evangelical Lutheran Church") is a collection of a number of written works, which correctly explain and summarize what the Bible teaches. Those who authored and collated these works did not do so in order to provide a new source of truth, nor to bring out doctrines never before revealed by God. Throughout the Book of Concord the reader/hearer is at all times directed back to the Bible as the only source for faith and life.
But they did understand the importance of confessing the faith taught in the word of God. Just as Christians of old summarized and taught this faith in creeds (which are included, by the way, in the Book of Concord) so they needed to confess the same faith at the time of the Reformation during the 1500's. They did so for a number of reasons, not the least of which was to contrast the Bible's doctrine with the false teaching promoted by many within the Church. But the main reason was to bring comfort and the certainty of forgiveness through Jesus to sinners.