Buddy and his friend make fruitcakes every year to give as gifts to people. Some are people they have never met, like the President of the United States, some are people they have met just once, like a couple whose car broke down in front of the house, and some are a peripheral part of their lives, like the knife grinder and a bus driver.
The fruitcake baking is an annual tradition that provides a tangible goal for Buddy and his friend. They contrive ways to save enough money throughout the year for the ingredients (dried and canned fruit, butter, sugar, spices, and whiskey) and stamps in order to mail the cakes.
The fruitcake baking is a way for them to make and maintain connections with others outside their rather isolated existence. It is also a way for Buddy and his friend to bond with one another.