Practice does not make perfect. Only perfect practice makes perfect." -Vince Lombardi
Vince Lombardi was one of the greatest motivators of all time. He won multiple Super Bowls, is a member of the NFL hall of fame and is still the standard for football coaches nationwide. The National Football League even named the Super Bowl trophy after him! When he made the above comment, it was most likely to motivate his players to work harder during practice sessions (in addition to leaving residual wisdom for later coaches. 'You'll play how you practice' is still a common theme in all sports). It is arguable that Coach Lombardi considered the science behind the above statement in regards to individual sport performance training.
When individuals perform a movement, the brain sends signals along a nerve pathway to the necessary muscles in order to successfully perform that movement. When signals are repeatedly sent along the same nerve pathway, the nerves become myelinated (insulated by a fatty sheath). Too keep it simple, myelination results in a faster impulse from brain to muscle. Therefore, it is imperative that technique for sport performance and sport-specific movements are practiced with technical precision and perfection. Any discrepancies in running, landing, throwing, kicking etc. will result in myelination along an incorrect pathway consequently causing muscle imbalance, movement inefficiency and/or an increased risk of injury.
The most difficult part of perfect practice is breaking old habits. Applying perfect technique to familiar movements can result in severe frustration as people often 'take 2 steps back to take 4 steps forward'. Whether the goal is to score more touchdowns, increase squat weight or improve sprint speed, the end result of hard work (as it often is) will be worth it.