In Spinelli's Stargirl, what is the purpose for the "Card Game" that Stargirl and Leo play?

In chapter 21, Leo discovers that Stargirl makes her own greeting cards. When she finds someone who needs a gift or a card, she anonymously gives them what they need. The way Stargirl finds out what people may enjoy for a gift or as a title of a card is through newspaper articles and public bulletin boards. When Leo gets to view some of her handmade cards, he notices that they are simple and hand drawn with stick figures. Although simple and not very professional-looking, each card is personal and heartfelt. This gives them more value than a card bought from the store.

Leo also discovers a game that Stargirl plays while in public. She follows and watches someone for fifteen minutes and then imagines what type of card she would send them. Leo points to someone and asks what type of card would she write for that person. This is how the game begins between the two of them. After following a woman around for that short amount of time, Stargirl makes up what she might say in a greeting card as follows:

"Hmm . . . to Clarissa I would send a While-you're-waiting-for-Harry-be-good-to-yourself card" (116).

Then Stargirl asks Leo to tell her what he would say to the same lady in a greeting card and he says, "I would send a . . . Don't-let-Harry-catch-you-flicking-card" (116) because he says he saw the lady pick her nose. Stargirl laughs.

The purpose of the game is partly a way for Leo to get to know why Stargirl gives gifts and cards to strangers. It is also fun to watch people and guess what they would need in their lives. Of course Leo wouldn't send a nose-picking warning card to anyone, but he takes something that his girlfriend likes to do and teases her by playing the game with her. In a way, Leo is better able to get into Stargirl's head and to learn about her by playing this card game. About the game, Leo says the following:

"And I did have fun. Whether it came from the game or simply from being with her, I don't know. I do know I was surprised at how close I felt to Clarissa and Betty and Adam after watching them for only fifteen minutes" (117).

Therefore, the game has a purpose unknown to Leo while he is playing it, too. He realizes that the game teaches him to step into someone else's shoes for awhile, consider life from their view, and to stop thinking about himself for a little while. This might be the first time that Leo learns how to be selfless, but it's not the first time for Stargirl.