Calpurnia serves as Scout's example of strong womanhood and femininity. Because both Jem and Scout are motherless (their mother died), Calpurnia helps fill that void as a mother-figure to both children.
Scout learns the joy and skills necessary for such "womanly" arts as cooking and even cleaning by watching Calpurnia, once Cal actually allows her into the kitchen. This occurs mostly around the time that Jem is hitting adolescence, when Calpurnia realizes that Jem needs "time to himself," and so she sequesters Scout in the kitchen with her to give Jem that allotted time.
Because Scout has no mother, and she runs "wild," as stated by Mrs. Dubose, Cal creates a sense of rules and regulations. It is not odd that during this time in the South the "help" would teach manners to the young and teach the basic principles of their culture (even if they were from different races). Many families allowed the nanny to raise children--and most were African-American nannies, or "mammies."
Calpurnia can scold and even spank Scout, which is something that Atticus rarely does. He explains the world, but he doesn't really have time for child rearing.