By changing the shock angles you are essentially changing the "moment arm" forces. Think of it as a torque arm. If your shock is perpendicular to the suspension arm the spring (and oil) will push aginst it with the greatest amount of force. When the shock is leaned inwards than you can split that force into horizontal and vertical components. Therefore your vertical resistance to the arm's motion is decreased. You also get the variables by how far in or out you mount the shocks on the suspension arm or the shock tower. So when you lean the shock inwards you are essentailly softening it so you should get more traction. So all these factors of having it just right helps tune the suspension.