This is a game , the main aim is not just to kill bosses but to do so effectively, and better each time.You could always over gear the instance and complete the raid much faster.
which dmage meters players can gauge their personal performance on a boss encounter.Id like to know how well i did after all it is a game like i said and part of the fun is doing better, progressing getting gear etc, i wanna see the improvements, don't you?
without meters you can't tell who is doing better, the boss will die, that's obvious but what helped the most?? or are you those "everyones a winner, we all get gold medals" type fellow?