In that the whole of To Kill a Mockingbird can be seen as a coming-of-age novel of sorts, Scout learns much from this short visit from Walter Cunningham.
Although less important, Scout learns that Walter is very well learned in regards to farming.
While Walter piled food on his plate, he and Atticus talked together like two men, to the wonderment of Jem and me, . . . expounding upon farm problems. (24)
The poor, then, are not always ignorant about absolutely everything. In fact, the poor can be incredibly adept at learning a very important trade, . . . it's just that the trade may not pay much money. A good beginning lesson for Scout to learn about the poor! Right at her level!
Most important, however, is the twofold lesson that Calpurnia teaches Scout (and Atticus approves with his various "head shaking" at Scouts comments and actions of the scene). First, Scout learns not to gawk at the poor, for we can never truly empathize with their situation. For, as Scout gapes at Walter for pouring syrup all over his food, she can't help but notice that both her father and her housekeeper react violently to her treatment of Walter at that point.
However, Calpurnia also teaches Scout a second lesson: to respect everyone who dines with them, . . . even the poor.
Don't matter who they are, anybody sets foot in this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty! Yo' folks might be better'n the Cunninghams but it don't count for nothin' the way you're disgracin' 'em! (24-25)
Or maybe it's more appropriate to say that, at this point in the book, Scout learns that Calpurnia respects everyone who dines with them. Scout is simply in the middle of the learning process and will eventually learn respect for each person, no matter what walk of life he hails from.