why do buddy and his Friend make fruitcake each year ?

I would say that there are two correct answers to this question.
They bake fruitcakes to hand out to "friends" as Christmas gifts.
They bake fruitcakes because it is a Christmas tradition.
Readers are told about the fruitcake baking tradition very early on in the story. We are told that it is November, and Buddy enters the kitchen. There is a woman standing there that sees Buddy and exclaims "it's fruitcake weather!" Buddy then introduces readers to who exactly this woman is.

We are cousins, very distant ones, and we have lived together—well, as long as I can remember.

Buddy's friend also claims that they have 30 cakes to bake. In the following paragraph, Buddy tells readers the following information.

It's always the same: a morning arrives in November, and my friend, as though officially inaugurating the Christmas time of year that exhilarates her imagination and fuels the blaze of her heart, announces: "It's fruitcake weather! Fetch our buggy. Help me find my hat."

It's clear to readers that Buddy and his friend have been keeping this fruitcake tradition for as long as Buddy can remember. To Buddy, Christmas time simply wouldn't be Christmas time without this annual event.
30 fruitcakes are a lot of fruitcakes, and it's clear from the story that Buddy and his friend love to do this project together every year; however, they can't possibly eat 30 fruitcakes themselves. They need some kind of reason to bake that many fruitcakes. Buddy tells readers this reason after the four days of fruitcake baking are completed. He says that the cakes are for "friends." The "friends" are not even necessarily good friends either. They bake the cakes in order to do a fun project together, and the cakes end up being a way to spread their Christmas cheer to the larger community.

In four days our work is done. Thirty-one cakes, dampened with whiskey, bask on windowsills and shelves.
Who are they for?
Friends. Not necessarily neighbor friends: indeed, the larger share is intended for persons we've met maybe once, perhaps not at all. People who've struck our fancy.