A: Oh, I'm sorry. I just realized that I forgot to bring the camera you lent me. I left it back in my dorm.
B: That's all right. I won't need it until tonight. As long as I've got it back by then.
请问这里的 left it back 为何加back呢?直接写 I left it in my dorm不就好了么,此处加back如何翻译呢