The higher your game shape is, the better your players will perform (more points scored, better defending, etc.), just like in real life.
Stamina is important too, especially for your starters. During the course of the game, your players will start to tire, causing their performance to drop. If a player has high stamina, he will either be able to last more minutes without receiving a penalty to his performance, or the effect on his performance will be very small. On the other hand, players with atrocious stamina will still be able to do well over a short period of time, but later on in the game their usefulness will diminish severely.