Its purpose is to level the playing field with a little more room for the attacking team to get the ball in play closer to the halfway point.  The defending team may not cross the build out line on a goal kick until the ball is put into play.

The lines are supposed to be midway between the penalty area line and the halfway line and extend the width of the field.

Explaining The Build Out Line for Youth Soccer - YouTube

Let us know if you have any questions.