Why did Sarah tell Jerome you're real?

In context, Sarah is acknowledging Jerome's plight, the death of a young man that is a result of racism. The fact that Sarah can see and communicate with Jerome and Emmett is a symbol of the importance of "bearing witness." In her afterword, Rhodes defines the term as meaning "using your personal and/or cultural story to testify against inequities, injustice, and suffering." Initially frustrated that he has to spend time with a privileged white girl, Jerome learns from Emmett that, as a living person, Sarah can affect change in the world in a way that ghosts like him and Emmett can't. However, they can share their stories of trauma with her, which helps them cope with their pain through catharsis. The ghost boys then empower Sarah to advocate for greater awareness of racial injustice.