Why does Grandma get samples of letters as a child in My Feelings?

In My Feelings, Grandma receives a letter from a prisoner in a Turkish labor camp. She becomes fixated on figuring out if the person is good or bad; so, she gets a letter from her father and a murderer at a prison where her uncle works. She cannot determine which handwriting the Turkish prisoner's letter is closer to, so she begins to get letters from other people such as the piano teacher, her grandmother, a classmate.