Children develop at different rates.
As a result we lose players to other activities. Competitive play can wait until age eleven or twelve. Playing small sided games allows younger players more touches on the ball, a less crowded field, more actual playing time, must play offense and defense- developing a more complete player. Soccer positions are dynamic- your position as it relates to the ball and other players. Youth coaches who attempt to teach strategy or team tactics are doing so in the mistaken belief that it will help their team.