In grouping, not all classes are similar. Some provide simple and easily defined benefits, while others provide auxiliary benefits beyond their basic stated role that more than make up for an apparent lack of output in that role. That was wordy, let's talk examples.
Rogues are pure damage dealers, so given the same skill of players and same gear level for all toons–(Rogue x5) should do more damage than (Cat druid x5), (Enhancement Shammy x5), (Beastmaster hunter x5), or (Fury Warrior x5). Rogue is a pure damage class, so this makes sense.
However, other classes bring valuable benefits in enhancing the performance of their allies in similar roles. Enhancement Shamans bring the almighty windfury totem. Fury Warriors bring Battle Shout. Cat druids bring Leader of the Pack. Beastmaster Hunters bring Ferocious Inspiration and Improved Hunter's Mark. All of these buffs work together.
So, a group that is (Rogue, Fury Warrior, Cat Druid, Enhancement Shammy, Beastmaster hunter) magically fit together like pieces of a puzzle and will do more damage than (Rogue x5). Their sum is greater than their components. The rogue will still top the damage meters, but that rogue will know that of their chart-topping dps, almost half of it is due to allied buffs, and far above anything they could generate without those allies. Enlightened raiding rogues love having an enhancement shaman and the others nearby, even though those friends do less damage than the rogue does. PvE is a team game, not a damagemeter race.
Likewise, (Mage x5) do great damage because they're a pure damage class, but (Mage, Warlock, Boomkin, Shadow Priest, Enhancement Shammy) does even more damage.
So group utility also plays into raid roles.
However, group utility is a strange thing. Most of these buffs are limited use only. So having two enhancement shamans side by side doesn't give you twice as much windfury benefit. Likewise with the cat druid damage buff and fury warrior damage buff. And so, like hybrid roles, there are a limited number of spots in raids for single-role utility/buff people.
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