The answer to this question can be found in Chapter 5 which is when O-Lan begins to use some of the best lard and flour to make some special cakes. When Wang Lung asks her what she is doing, O-Lan explains that she is making these special cakes with the best ingredients in order to take them as a gift for the Old Mistress at the House of Hwang when she will present her son to her. This is something that she told O-Lan to do, and, in a culture where appearances are so important, it is a great status symbol to be asked to come and visit such high class people as the Hwangs. Note how Wang Lung responds to this:
Then the cakes were more important than ever, and Wang Lung was pleased that to the great hall where he had stood with so much timidity and in such poverty his wife should now go as visitor, carrying his son, dressed in red, and cakes made as these were, with the best flour and sugar and lard.
The visit is important because it therefore gains prestige for Wang Lung and his family.