It’s just to call out that Supergirl is going to have lots of room to grow.
Basically the Superman film is going to have an established Superman, no origin story (although I’m sure sprinkled in). Supergirl is going to be more of an origin story (as I read it).
It will be the job of Superman to teach Supergirl who is coming from a very different background.
Remember the scene from Suicide Squad where the Ratcatcher father is telling Ratcatcher daughter about rats? Swap in Superman and Supergirl and change rats to humanity, boom. That’s the kind of wisdom Superman is going to be dishing out.