In the league I am better than any team and don't need to spend it, in the cup there might be 1 team better, CL there is 1 a lil bit better, and him.

Why I don't spend the SP. Here I quote dv8r.

Once your player hits the next star, their progress level will decrease because the player is considered a developing player by the game engine. So your best bet is to stockpile your skill points until you're finished.