Star level increases the amount of points girl gets after practice.

10 hour practise:
7 stars (Dora) - 1381 points
30 stars (Cherry) - 1977 points
36 stars (Geeky) - 2133 points
59 stars (Lola) - 2729 points
103 stars (Sanshi)- 3870 points
139 stars (Ice) - 4804 points
Maybe it also depends on some other things, but right now it looks like the higher the amount of stars the more stats they get when practicing.

Unlocking more girls allows you to unlock their Halloween outfit while the event is running (if you get 25 pumpkins). Outfit bonus is 33% to all three stats.