Why mark targets?

[AniCelestine]

Also, your second graphic is actually showing the health bars of the enemies remaining on the field - not the threat.

Well, I at least have the red square, yellow thing, green things( can't remember which are which shapes) which indicates if I have the aggro of the monster or not(red= I have aggro, orange/yellow I'm nearly having the aggro, green = I'm nowhere near of having the aggro) but that's what I use more on a AoE dps so I know can I throw another AoE or should I do 1v1 skills on some monster. But the pic looks odd to me, since the red marks aren't squares, the are more of a lines <.<