being able to play out from the back is important for development because it forces all of the players, defenders, midfielders, and forwards, to learn to be good technically with the ball at their feet - good first touch, first time and two-touch accurate passing and to be good tactically - good decisions, good spacing, knowing your options, switching and combining, taking advantage of 2v1 situations, etc. Also, youth goal keepers can't just send everyone "up and in" and punt the ball very far until they're teenagers. I don't even think goalies are allowed to punt until U13 or so when they start 11v11. If you can't play out from the back and depend only on long balls and counters for any dangerous possession opportunities, players will get more gassed, it can impede players' technical development, and what can end up happening is the product looks like kick-ball - not soccer. You have to be able to mix it up and do both, adjust to what your opponents are giving you in each situation, and adjust to your own teams' strengths, etc. If your keeper or defenders want to just play kick ball and skip the midfield every time, can they get it there proficiently? Can your forwards win a 50-50 ball? If they have to check to the ball, you want to end up with them getting a diagonal ball going to goal or get someone overlapping wide so they can crash the box, so can they one-time the long ball to someone to set up that run? It's all important to work on and players have to be good and be able to play in all different situations.