The answer to this question can be found in Chapter 24, which is where the missionary circle meet. Ironically, just before Atticus bursts in, the white women show their hypocrisy through their concern and compassion for African tribes far away from them but their racist attitudes towards the black servants they have, who they say have been playing up since Tom Robinson's trial. It is at this point that Atticus bursts in to take Calpurnia away. The reason is because Tom Robinson had just been shot:
"They shot him," said Atticus. "He was running. It was during their exercise period. They said he just broke into a blind raving charge at the fence and started climbing over. Right in front of them—"
This is of course a tragic event, but the fact was the guards shot him seventeen times, which is excessive, as Atticus himself notes. Atticus wants Calpurnia to go with him to go and tell Helen, Tom's wife, about this sad news. This chapter therefore focuses on the hypocrisy of white society and their ignorance of how blacks are being treated under their own nose before a tragic example of that very fact is introduced yet again.
Calpurnia is the bridge between the white and black world for Atticus' children in the story. She also serves as bridge in her own black community for Atticus' motive in the case. She is a constant within the story and an appropriate help in delivering the devestating news of Tom's death to his family.