Having an inner voice makes it easier to absorb the information in a book
I think all of our brains are wired different and the different wiring leads to advantages in one thing but it's probably a disadvantage for others. For instance I have no inner voice but my reading speed, with comprehension, is well faster than nearly anyone I've ever met. I can even sometimes recall precisely where on a page a given word or phrase was located, even years after reading the material. However I'm almost entirely unable to imagine a 3 dimensional object and rotate it in my "minds eye".