When you are baptized, it is a symbol of what God has already done for you in making you a part of the Church family. Even though you are a part of the family, you are not yet expected to be a fully engaged member of the church. For example, when a child is born, they are not immediately given a list of chores that they need to do. Later when they get to an appropriate age, they are expected to contribute their part.
The same is true in confirmation. By being baptized, you are already a part of the church family. Many times, people are baptized without the ability to make their baptismal vows for themselves. That is where confirmation comes in. Confirmation provides you with the opportunity to renew your baptismal vows, and thus become a member of the church.