We are never given the precise reason for the flurry activity that starts each November as Buddy and his friend make fruitcakes and gather the necessary ingredients, so we are left to infer the reason behind this Christmas traidition from the text. Clues can be found that help us to do this. For example, we are told that the "fruitcake weather" that starts off this activity "inaugurates" Christmas and "fuels the blaze" of Buddy's friend's heart. In addition, we are told that the cakes are for people that they have either met only once or not at all, as Buddy's friend is very shy. Note what Buddy himself deduces about the way that they give cakes to people they hardly know at all:
Is it because my friend is shy with everyone except strangers that these strangers, and merest acquaintances, seem to us our truest friends? I think yes. Also, the scrapbooks we keep of thank-you's on Whiet House stationery, time-to-time communications from California and Borneo, the knife grinder's penny postcards, make us feel connected to eventful worlds beyond the kitchen with its views of a sky that stops.
This quote introduces a secondary motive: the world of Buddy's friend is very limited and is defined by the "sky that stops." Giving cakes each year to so many different people means that Buddy and his friend are able to feel connected with a bigger world beyond the vistas of their lives and experience.