The magic was part of the reason she wanted to go to Hogwarts. Mostly, though, it was for Lily.
Lily was impulsive. She acted before she thought. She ignored propriety and made enemies. Petunia knew Lily needed looking after, and that Snape boy could only make things worse.
When she reads the letter tucked into her nephew's blankets, Petunia curses Albus Dumbledore. He should have let her guard Lily. Maybe then Lily would be alive to raise her own son.