Why the first council of Nicea was called?

The council of Nicea was called to settle a dispute in beliefs that happened between the Church of Alexandria and other churches about the nature of the Son of God. Some early church bishops believed that Jesus was created by a more powerful God while other Bishops took the opposing view.

Arians believed that Jesus was created by a God and and that he had a beginning. This means for them that he was less divine than the father. The rest of the council , according to Wikipedia, believed that Jesus was begotten and so , for them, he is considered as divine as the father.

One of the purposes of the council was to agree on a unified date for Easter that all Christians can agree upon and celebrate. 

The Roman Emperor Constantine I was the one who called for the council of Nicea. It's believed that his motive was to prevent the empire from being divided as opposing views lead to conflicts between Christians.

According to Wikipedia most Bishops thought that the teachings of Arius were heretical and dangerous to the salvation of souls. Hosius of Córdoba , the Bishop of Córdoba, recommended to the emperor that  council takes place to resolve those beliefs. (See How Wikipedia Started?)

Meletius was a Bishop in Egypt who founded the Christian sect called the Meletians who had some conflicting beliefs with other Christians. Meletius was seen as a disturber of the peace of the church. The council aimed to make peace with Meletius. (See Why is Christianity popular in Europe?)

The council discussed the Liturgical practice. This included the place of deacons, and the practice of standing at prayer during liturgy. 

According to Wikipedia the council also addressed the issues of ordination at all levels and of suitability of behavior and background for clergy.

The council addressed the reconciliation of the lapsed and established norms and rules for repentance.