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Casters also get proficiency up, that depends on total character level.

And higher spell slot levels still increase the character's damage output and specific utility. Some spells - such as Cure Wounds or Hold Person - are pretty much designed to be upcast, and some others (e.g. Heat Metal) are unexpectedly great. Someone with Bard 3 / Sorcerer 4 might only have second level spells, but a level 4 Heat Metal vs. a heavily-armored target deals 4d8 per turn without a saving throw for up to 10 turns.