AFAICT the bars add a kind of "laser" that immediately inflicts damage on all bricks in your initial selected trajectory (penetrating all bricks that intersect the dotted line representing your initial trajectory). The more bars you collect the more damage this laser does.

This provides a counterbalance to only blindly selecting the trajectory that has your dots hitting the most bricks. Sometimes you're willing to have a poor trajectory that results in your dots immediately hitting the "ground" after only one or a few bounces if the "laser" can do enough damage, or sometimes the laser can "carve" a better path.