Each of these functions consumes a data item that describes an unknown number of balls. In conventional languages, our first encounter with arbitrarily large data is about files. We read items from a file until no more items are left. This kind of function, however, involves io, which is what we don't want. Lists will play an analogous role in our world, but they are different, and because they are different, they generalize to other forms of data.
Structures are one way to represent compound information, and lists are another.
Forming lists is something that all of us do. Before we go grocery shopping, we often write down a list of items that we want to purchase. When we plan out a day in the morning, we write down a list of things to do. During December, many children prepare Christmas wish lists. To plan a party, we enumerate the people we want to invite.