It’s a holdover that doesn’t work great.

It... Does. I enjoy having spell levels and slots way more than something like mana. In any system with mana I almost always use either the most dmg/mana efficient spell or the big one, etc. Having spells be divided in levels and action/bonus action makes for a much more interesting combat.

For example, I much preferred the way DS1&2 does spellcasting compared to DS3 and elden ring. You have a limited amount of casts and the system forces you to either have a varied spellcasting repertoire or to find your way to get several of the same spell to ge more uses. Mana is... mana. Boring.