The biggest advantage of classes, IMO, is the power of names.
Compare, "I took an 8th level Ranger through Keep on the Borderlands" with "I took a 50-point fire/summoning focused character on the GM's custom war politics planar adventure".
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Character creation is a fun minigame for some. Optimization is a fun minigame for some. The existence of playing pieces of inherently different strengths allows the optimizer enjoy their optimization minigame without producing an overpowered character.
Personally, I'd almost be happy if the entirety of character creation involved me writing, "Quertus, Wizard" on a character sheet. Or, rather, if it could be that simple to produce a viable character, but the option existed to create characters of increasing complexity as desired, to a) better match the mechanics to the character concept; b) to allow players to scratch their optimization itch; and c) to increase character differentiation / replay value.
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I prefer calling nonmagical beings "muggles". "Mundanes" also works, I suppose, but I view it as an insult.
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So, RPGs shouldn't have big guns?
2e produced the most units of "fun"; therefore, 2e is objectively best RPG ever.