Why focus below your bellybutton in meditation?

My roshi tells me to focus here. Before practicing, I spent most of my waking hours thinking, analyzing, living in my head. With practice, it has gotten easier to think when thought has a purpose, and to listen to my body and environment otherwise. The book that introduced me to zen said to picture your breathing below your bellybutton. A friend said to focus your energy there. As I build focus here, it is easier to let go of distraction and delusion. But why is this place significant? Why not focus on my chest of elbow or toes?