Last week back when my raid was practicing T8, we pug a bard who wear Animus and 4 high allagan pieces.
We assigned him only HM+BJ and one tower. Global dps was a bit slow as we barely kill it when Avatar cast Atomic Ray at 60% cast bar (1 sec)
What we saw from parser is other dps dealt 300+ dps but him only did 230 dps.
Without parser, who think he was holding our global dps back considering to his gear ?
Additionally, with parser, we know he used Straight Shot as main skill, didn't keep DoTs uptime, barely used Blunt Arrow and Flame Arrow.

We are not telling him that he was under performing since we just wanted to learn mechanic but fortunately we made it.
This week, he is asking to pick him up again for T8. Instead of deny him, we will give him some friendly advise to make our task easier.

TL;DR : Better gear doesn't mean better dps, parser can tell. And we can use parser to improve community.