When blocking with a weapon it only absorbs some of the damage of an attack and is ineffective at blocking projectiles. With a ranged weapon such as Jarids, or a Bow, it shows the place where the arrow/weapon will go, the wider the reticle the less accurate the Bow and throwing weapons are as they can go anywhere between the reticle lines.
The targeting reticle shows where your attack will land with a melee weapon.
