Grimoires are needed to use Ars Magus

A grimoire was a key necessary to use Ars Magus, and although it was called a book, it didn't necessarily take the form of a book. Ragna's arm, while taking the form of a human arm, was said to be a grimoire, not an arm.

It was too difficult a story for Ragna, who didn't understand the concept of Ars Magus in the first place, but the precautions that Rachel and Jubei repeatedly said were ingrained in his head, beyond understanding.
- Blazblue: Calamity Trigger Part 2 Chapter 3