There's nothing we love more than closing the loop, as far as a story is concerned. The story could have ended with a happy Sara frolicking through the London spring with blossoms falling on her pretty head, Mr. Carrisford laughing indulgently at her side, and Becky gasping with glee (people gasp a lot in this story), but it doesn't.
Instead, it ends at the bakery, which we'll recall is a place where Sara experienced a bit of a low point in her life (you know, the whole starving and freezing to death thing). When she first enters the shop, as a poor scullery maid, it's pretty sad:
She went into the shop. It was warm and smelled deliciously. The woman was just going to put some more hot buns into the window.
"If you please," said Sara, "have you lost fourpence—a silver fourpence? And she held the forlorn little piece of money out to her. (13.35-36)
But at the same time, the bakery is where Sara realizes that she has a choice. Even starving to death, she has the choice to give that cold hungry child some of her food. And this realization sticks with her. Rather than forgetting about her misery, she remembers and returns to the bakery, offering to buy food for all of the hungry children.
When she meets the girl who was the child she gave food to, she recognizes that they aren't that different after all. Look at the way the moment is described:
Sara took her hand out of her muff and held it out across the counter, and Anne took it, and they looked straight into each other's eyes. […]
"I am so glad," Sara said. "And I have just thought of something. Perhaps Mrs. Brown will let you be the one to give the buns and bread to the children. Perhaps you would like to do it because you know what it is to be hungry, too." (19.43-44)
This ending is way more meaningful than just the "I've made money, hooray!" narrative, because it shows that Sara has grown as a person. Now, she truly knows what it's like to be one of the populace.