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.
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.