I see the bananas as something longed for which shows the limitations of the Bundren's imagination and/or aspiration. Anse only longs for teeth and a new wife, Dewy Dell not to be pregnant and Vardaman for his mother to be alive in some context. They all aspire for what they already have had. The bananas are a safe treat as they know what to expect.
I think the bananas are a treat for the family and so demonstrate how poor and truly provincial the Bundren's are. Though this is not an especially significant detail, as far as I can tell, it does work to suggest that the family is quite poor - so poor that when they splurge, they splurge on something as cheap as bananas.