What is the significance of the Nicene Creed?
Not precisely. First the Apostle's creed is also accepted by all Christian denominations. Second, there are several groups that broke off from the main body of the church because they did not accept the Nicene Creed. Third, you are confusing the Athanasian and Nicene formulations of the Trinity. The Nicene Creed was responsible for the homoousia formulation, but left many other areas of Trinitarian theology untouched.
The main significance of the Nicene Creed was that it established much of what is now known as orthodox Christian teaching on the subject of God and the Trinity. It remains the only statement of faith that is accepted by all major parts of the Christian faith.
The major point of the Nicene Creed is that it defines what the church believes about the nature of God. It establishes the idea that all Christians must believe in the idea of the Trinity. It establishes that God the father, Jesus, and the Holy Spirit are all part of one being. They are separate beings but somehow one. This is a central "mystery of faith" that underlies all of Christianity.
Everything that pohnpei397 is true, but I have a minor detail to add.
The Roman Catholics have added so that it reads: Who proceeds from The Father, And The Son.
The Protestants have changed it so that it reads: In one Catholic and Apostolic Church.
The Orthodox have the original: Who proceeds from The Father.
In One, Holy, Catholic, and Apostolic Church.