I don't have access to a specific passage that discusses The New York Historical Society or its mission. However, I can provide a general answer to your question.
Teaching people about slavery in the North is important for organizations like The New York Historical Society because it helps to provide a more accurate and complete understanding of American history. While slavery is often associated with the Southern United States, the institution of slavery was also present in the North, including in New York City, which was a major hub of the transatlantic slave trade.
By educating people about the history of slavery in the North, organizations like The New York Historical Society can help to dispel the myth that the North was somehow immune from the evils of slavery. This can help to foster greater awareness and empathy for the experiences of enslaved people, as well as help to address the ongoing legacies of slavery and racism in American society.
Additionally, understanding the history of slavery in the North can help to illuminate the broader social, economic, and political structures that supported and sustained slavery in the United States. This can help to deepen our understanding of the complexity and nuance of American history, and help us to better confront the challenges and injustices that still face us today.