Given the subjects of the rest of your questions, I assume that you are asking about the First Council of Nicaea. I have edited your question accordingly.
The main reasons that this council was important were that it led to the rise of papal power and that it helped to lead to the creation of an orthodox version of Christianity embodied by the Church. The coucil was called mainly to decide between the Arian doctrine and that of Athanaisus. The two disagreed as to whether Jesus was both God and human. The council ruled in favor of Athanasius and Arianism became a heresy.
By highlighting the differences in opinion among Christians, the council helped to create the idea that there needed to be one central authority on religious matters. This helped to allow the bishop of Rome to become the acknowledged leader of the Christian Church.