@MRain
Even in Tabletop RPGs, Tanks are a super helpful class.
I'm not much of an expert, but talking to some of my friends who are seasoned vets in tabletop RPGs, the biggest thing is that the tank can endure damage, which is super important when faced with an enemy that can potentially do a party wipe.
Sure, there are classes like the paladin that can do double duty, but they are still a tank though and through.
Last point, MMORPGS were inspired by tabletop RPGs, so why would the roles of tanks would be any different.