The purpose of the collection is to aid Tom Robinson's wife while he's in jail.
When Mayella Ewell accuses Tom Robinson of rape and he's put in jail, it makes things difficult for Helen and their children. Without Tom, the household doesn't have as many resources.
Calpurnia takes Jem and Scout to First Purchase African M.E. Church with her one Sunday morning where they attend church services. When Reverend Sykes is making announcements, he says:
"You all know of Brother Tom Robinson’s trouble. He has been a faithful member of First Purchase since he was a boy. The collection taken up today and for the next three Sundays will go to Helen—his wife, to help her out at home."
When the collection is gathered in the black enameled coffee can, Reverend Sykes says it isn't enough. He reminds everyone that Helen can't leave her children to work and says they have to make at least ten dollars. He instructs someone to shut the doors and says that no one can leave until they've reached that amount.
When Scout is leaving, she asks Reverend Sykes why Helen can't take her children to work with her when so many other women do. He says that Helen is having trouble finding work. Calpurnia tells Scout that because of the crimes of which Tom's accused, people are not associating with the Robinson family.