The tension tie prevents the outer ends of the beam from spreading. The house keeps the inner ends from spreading. If you push the rafter pair in middle, down with a given force, they will push the 2x8s outward. Think of it as a plank. Put a 2x8 on the flat, across 2 sawhorses 5 ft apart, and walk across it. It bends right? Now put 2 in a sandwich and block it in the center, and every 12" .. walk across it. Wont bend. Turn it 90 degrees, now You made a truss for sideways force. Science, bitch.