I am currently in the history department at Yale, and I have a few thoughts.
1) I think the faculty is fairly split on usefulness of this assignment. Some think it doesn't tell them anything not found elsewhere in the app, others like it.
2) Write about the book you most wish you had written (ie, one whose topic and methodology you enjoy/admire).
I don't remember the exact prompt. Are you just supposed to write a book review, or are you also supposed to discuss what your choice says about you as a historian?