@Dushanan
That's a direct reference
to the Nino/Erk supports in the original game.
Nino doesn't know how to read, she is a natural at magic, though, so she managed to learn by imitating her "mother", Sonia.
Sonia did hate and abused the girl, so never actually attempted at improving her literacy.
As a side note this support was in turn a throwback reference to a bit of Akaneia/Jugdral FE Lore: wizards requires grimoires as magical tools to cast spells without exerting their life force (unlike i.e. Valencia's casters), but aren't actually required to read during combat, they just need to know how to cast the spell and draw power from the magical book.
Each grimoire still has the specific spell written on, until its power is entirely consumed and change back into a blank book, so learning from the grimoire it's the most obvious way to master the spell.