Severus Snape knew many things, not that he was one to brag about his knowledge. He knew how each and every ingredient would react to another or to any outside stimulus whether that be heat or a stirring rod. He knew how to fight and how to create spells to end the fights quickly. He knew how to deceive even the most paranoid of people. He knew that he should never have agreed to become a teacher and, above all else, he knew that Petunia Dursley nee Evans should never be allowed within a hundred feet of a magical child.
A Severus raises Harry fic because I love them. Simple as that.