Could not have said it better:

… the practice sessions of aspiring champions have a specific and never-changing purpose: progress. Every second of every minute of every hour, the goal is to extend one’s mind and body, to push oneself beyond the outer limits of one’s capacities, to engage so deeply in the task that one leaves the training session, literally, a changed person.

Purposeful practice is about striving for what is just out of reach and not quite making it; it is about grappling with tasks beyond current limitations and falling short again and again. Excellence is about stepping outside the comfort zone, training with a spirit of endeavour, and accepting the inevitability of trials and tribulations. Progress is built, in effect, upon the foundations of necessary failure. That is the essential paradox of expert performance.

Syed, Matthew. Bounce: The myth of talent and the power of practice. Fourth Estate, 2011, p. 74 and 79.