This is not a high level nuance, it starts as low as 1st level with spells from origin feat, species and class.
Further, it is fundamental to the game when you consider multiclassing for two reasons:
1. The number of spells you prepare and which spells comes from each class individually. If you are not tracking your Warlock, Paladin, or Arcane Trickster how can you determine whether or not you can prepare Protection from Good and Evil or Compelled Duel? The character I linked below has 13 1st level spells prepared coming from 5 different sources.
2. Your saving throws and attack rolls are tied to specific ability scores. If I am not tracking whether Truestrike is from my Arcane Tricster (8 Intelligence) or my Warlock (20 Charisma) how do I determine whether it is -1 or +5 to attack?
Actual example from play, the Warlock-Paladin-Arcane Trickster below is being played and has a 20 Charisma and an 8 Intelligence. If I don't have to track which spells are from what source I can get much more effective combat spells from my Arcane Trickster subclass. It is a big difference.
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