It gives advantage. I’m not sure how that helps you cast shield or enlarge person. If True Strike is a buff, then it can’t give you advantage. That’s how I read it anyways.
It is pretty straightforward. I don't know how it helps for Shield, but it definitely helps on Enlarge/Reduce.
If you are a 3rd level Sorcerer with a 20 Charisma your DC on the Constitution save is 15 for Enlarge/Reduce. If you activate innate Sorcerery first it is 16.
I don't read that Truestrike is a buff at all, especially since it does nothing for you personally.