OK, people have been saying "cooperative game" like it absolves a game from needing to be balanced.
In a cooperative game, you don't win or lose individually, it's all or nothing, everybody wins or loses based on the sum of their efforts.
D&D is a cooperative game. OK, with the exception that individual characters can very much lose by, y'know, dying, or arguably "win" by getting the best magic items or whatever.
Early D&D could be more of a hybrid, rivals cooperating to survive, but still competing for certain things.
But, modern D&D, yeah, meant to be cooperative.
Thing is, individual accomplishment in a competitive game - the point and a big part the game - is simply winning. The most important thing in a competitive game is thus fairness - everyone has an equal opportunity to win if they can bring the skill & luck. Not so much balance, what does it matter if some choices are better than others, if you all have access to the same choices, you can each pick the best ones and have a fair competition. Knowing which ones are best? Player skill!, also part of the competition.
But, in a cooperative game, the gauge of accomplishment, the point and a big part of the game is contributing. And it's not just for the fun & feelz of each individual, it's for the win, too. If you under-contribute, you're dead weight, do so too badly, you'll lose the game for everyone.
Balance is thus critical for a good cooperative game. All the moreso if part of the idea is a heterogenous team that each steps up and contributes differently, rather than just a collection of best classes with the maximum number of best spells at the ready.