My theory is 2 fold:
1. As mentioned, better alignment.
2. The longer & slimmer the screwdriver the more torsional energy it stores.
So you are able to build up torque more gently. In addition when the screw does let go, 'stick / slip' comes into play allowing the screw to loosen more in the first movement. There is a much more noticeable 'crack' as it lets go.