If you've ever watched the Death Star try to get in Firing Range, then you already have a basic understanding of what Line of Sight is (LOS). The Rebel Base has aggrod the Empire and the Death Star just wants to eat it for lunch but that other darn planet is just in the way! Ok, maybe that is a galactic stretch of an analogy.
Whether you've got the means to take it to them, or drag them to you, or silence any enemy healers at ranged, there's still plenty of situational things that can happen where being able to manipulate mob positions based on your terrain can be quite advantageous and give you that edge other tanks may lack. Remember to communicate to your group about a LOS pull., DPS love to begin, often when you're not quite ready,
There isn't always terrain or an object to force these mob types into a location. If you're in this scenario always bring the Melee mobs as close as you can to the Caster/Ranged mob. However, next time you have a chance and there's a stairway, wall, corner or some other kind of structure that can't be cast through, give it a go. Use the Los!