Cal does just about everything. She is officially the housekeeper, but Atticus notes that they could not get along without her. She is strict but caring. She tries to raise Scout and Jem as if they were her own children. She is their surrogate mother. When Aunt Alexandra wants to send her away, Atticus refuses.
Scout often thinks that Calpurnia treats them unfairly or is too harsh. Atticus assures her that she needs to mind Cal and Cal has important things to teach them. So Cal does much more than cook and clean.
At one point, Cal takes Scout and Jem to her church. In the story, this serves an important function. It gives us a window into another culture in Maycomb. It also gives Scout and Jem a place to view the trial, because they sit with the Reverend in the balcony.