The point in the a* is that people would often do very little work in the a2 year because there AS was so high, and those that do aren't recognised.
I know someone with 297 at AS, so he would need 183/300 next year to get the A. However to get the A* would require 270 which gives him a target AND a reward and recognition for his work.
Without the A*, if he does as much work and get 270+ next year, he will not be rewarded for that work.
Basically the A* is an addition to the A to recognise hard work in the A2 year which would otherwise go unrewarded.