What's with the level 11 tank ?

I was doing a raid a few weeks ago and there was a big argue about level 11 tanks, a group of people saying it's the best way to do timewalking dungeons and a group saying this ruins dungeons. I didn't get it, as I don't know much about tanks. But it made me keep an eye out and I realized most timewalking dungeons I was doing at the time had a level 11 tank.

Seriously, what's up with that ? What massive gameplay difference does that cause for the rest of the team ?