There is a variety of reasons why private practise is both, necessary and essential in the development of future athletes. In this matter it is important to state that nothing can replace team practises, but some more hours with the ball can make the difference. Researchers have found empirical proof that it needs up to 10000 hours of training to make the development from a talented player to a professional. That's why I believe in getting more touches on the ball in extra hours.
Advantages of private practise sessions:
10x more touches on the ball than in regular practise
up to 500 touches per session
Individualized training plan to work on player's weaknesses