One of the things I hated about the book, was how many times Snape was a jackass to them. But then all the sudden he magically becomes Harry's protector
Snape as an asshole to Harry because of his relationship with his father and how he saw him when he looked at Harry and the resentment he felt for Harry's mother's death. However he always protected because Dumbledore asked him and Harry was needed to defeat Voldemort (in book ONE for instance he prevents Quirrel to make Harry fall from his broom to a certain death).
It literally means pointless