Her magic trick is to spout spells backward. "Freeze in place!" becomes "ecalp ni ezeerf!" Bad guys can incapacitate her with a muzzle, and she's helpless. I'm sure there's a feminist comment there, like when Wonder Woman loses all her powers if her wrists are shackled. In recent years, she was retconned as a childhood friend of Batman, and that gave her a rationale to branch into meager detective work.
Zatanna's costume is a stage magician outfit, a big improvement over her original, vague, flowing hero drapery. That was essentially a nightgown, conveying, I dunno, mystical regalia.