It still helps. It allows the section of the through/left lanes that come after the right turn lane separates to be full of cars going left and through (instead of having the turn-right cars sprinkled through). That increases the throughput of cars going left/straight because: 1. the cars turning right aren't competing with them to go straight 2. the cars turning right are slower to accelerate and clear the way then cars going straight/left.
Edit: Maybe a better way of saying it is that in the current set-up, a small gap could let three cars go simultaneously. 2 left/through and 1 right. But if you didn't have that right turn lane, a small gap would only let 2 cars go.
Edit: Maybe a better way of saying it is that in the current set-up, a small gap could let three cars go simultaneously. 2 left/through and 1 right. But if you didn't have that right turn lane, a small gap would only let 2 cars go.