When I first began writing my SOP, I didn't really know how to structure it or even what to write. I looked at some samples and that helped. One sample began with stating outright what your "purpose" is: "I am applying to a PhD program in X at Y because Z." My mentor/advisor told me that the admissions committee already know that you're applying to that program, and since they're reading hundreds of them, it would be better to start with something interesting. He suggested that I start with a quote (based on a primary source used for my MA thesis), and then introduce/connect my research interests based on that quote. It definitely sounds better than the straight forward approach, IMO.
I wonder what everyone's opinions are on ways to start a SOP.
P.S. My advisor is about two years out of a PhD, and he became an assistant professor right after finishing his PhD. He is young, so I feel like his advice is the best I can get, for me, since I'm following his path. However, that's not to say that other professors with more traditional approaches or advice aren't helpful. I'm open to everyone's input!