Why does the party need to be balanced?

[Ignimortis]

But fundamentally to me, this just reinforces the importance of balance; 5E is better balanced* than 3.X, making it harder for classes to inadvertently outshine another one. (Or "deliberately", at least not without a higher degree of "systems mastery". )

(*This isn't necessarily without 'tradeoffs' though. To me, 5E achieves this by limiting options [sometimes with arbitrary-seeming limitations] and with a more 'vague' ruleset that necessitate players asking for DM permission to do something that is either not present or not clarified clearly in the rules. Which, well, isn't necessarily bad [it's kinda like older editions, in a way], but a tradeoff.)

Take note that 5e is also balanced around Fighters. In 5e, a reasonably-built and played fighter with a +1 sword and no other magic items can still contribute a lot against 99% of the monsters in the game, unlike 3.X, where high-level monsters were usually far more fantastical than the default Fighter. Some people like it, the others point out that this design decision turned most enemies into sacks of HP and damage with little individuality ability-wise.