As DPS, we have one job, to do damage. You can tell a bad tank by mobs hitting everyone else. You can tell a bad healer by people not getting healed... DPS, you can be carried and nobody will ever know. Then you have timed parts of fights. And you keep failing. Why? Well, clearly at least one of your DPS isn't doing the damage you need to be able to make that kill... But let's keep holding hands and banging our heads against the wall.

Parsing damage lets you see where your weak points are, where you need improvement. And the competitiveness of it tends to make your better players play even better.