The Creed provides a "skin" for the church.
A skin's a boundary that we live in. So the Creed.
The Creed both widens and narrows the limits of what we can believe. Most of us are wanting to narrow definitions, and limit the true faith to those who believe and teach as we do.
For example, I'm a 6 day creationist. I think this is pretty plainly taught by the Scriptures, both in Genesis and throughout the Old and New Testaments.
However, I can't say that someone who disagrees with me is not a Christian. The Creed is plain: "I believe in God, the Father Almighty, maker of heaven and earth." As long as someone acknowledges that God is the creator, they are in the "skin" of orthodoxy. I might think they are wrong on details. And I can argue with them. What I can't say -- by the Church's definition in the Creed -- is that they are a heretic.
There are a number of things left somewhat open -- and thus "broadening" -- within the Creed: details on the inter-relations between the members of the holy Trinity, details of the creation, the nature of Christ's becoming man, the length and extent of punishment for sin, and the nature of the after-life, among others.
Others are narrowing. I think especially about our acknowledging of "one Baptism for the remission of sins." I think -- given that clause -- it's important to point this out to some of our brethren who "re-baptize" Christians who were baptized as children. Following St. Paul ("one Lord, one faith, one Baptism," Ephesians 4.5), the Creed forces us to acknowledge Trinitarian Baptisms, even when we're not comfortable with the circumstances or timing of the Baptism.
Without our skin, we would die. And without the "skin" of the Creed, the church dies.