Atticus asks Calpurnia to accompany him to the Robinson home because she knows Helen Robinson as a member of her church, the First Purchase African M. E. Church, and may be able to better comfort Helen.
In Chapter 25, Atticus Finch learns that Tom attempted an escape because, as Atticus tells his sister Alexandra after interrupting the ladies's Missionary Tea, Tom was "tired of white men's chances and preferred to take his own."
As Atticus and Calpurnia head to the Robinsons' home, they see Dill and Jem walking back from Barker's Eddy, where they have been fishing. They wave at Atticus; he slows down and tells them, "You better catch a ride back. I won't be going home for a while." But, Jem protests, so Atticus lets them enter the vehicle. Later, Dill relays what happened; he tells Scout that Helen fell to the ground when Atticus told her about Tom's death, "like a giant with a big foot just came along and stepped on her." So, Calpurnia stays with her to help out with the children and offer support.