Why the foot is kept in 70-degree external rotation or abduction?

The foot is like a boat. It has three movements that happen simultaneously. Yawing, pitching and rolling all three movements happen at the same time in boats. The foot also yaws, pitches and rolls. So it is not enough to get the foot that was inward and upward straight but a 70-degree external rotation is required and this is achieved during the casting and tenotomy to ensure all three movements.