Most bosses have lethal mechanics where you need to be able to see their target ring. (Examples: Scrapline, Radiant Sacrament, Biting Halberd, Left/Right Laser Sword, Shadethrust.) Tanks/DPS usually already have the boss targeted, and will have no issue.
As a healer (particularly the main healer), you often need to target a party member to heal them. While doing so, you can't have the boss targeted (without mouseover macros).
This means that healers who want to do mechanics correctly are forced to change targets to the boss and back, even in the middle of heals. I know that some more-casual players just guess, because they did not want to untarget their tank.
I personally think it would be a massive quality-of-life change for healers and others if we had the option to make our Focus Target's facing indicator / target ring visible at all times. To avoid issues, it could be given a different appearance, custom color, and the ability to be toggled off.