| Damage meters are used in the world of video games to depict the amount of damage a character has taken as a proportion of the total amount they can take. This is most often used on enemies and particularly end of level bosses to give the player an idea of the scale of the task ahead of them. It is basically the enemy's equivallent of the player's health bar.
The device is useful in situations where the character itself can't be modified to readily show it taking damage. (For example, memory or computational restrictions may limit the amount a character can be visibly distorted.) Judging how much of the task is left can also be difficult if the enemy is abstract and the player has never seen a half dead, mutant, baby, underwater, robot squid of peril before, let alone tried to defeat one in an R9. | 
