I have both forms. The inner monologue voice is a common learned way of thinking. For me it’s a way of testing how things sound, before using it in public. It also formalises ideas for memory.

Below that, I have my mindstream. It’s the active amalgamation of ideas, images and concepts that forms my intellect. It’s difficult to map to language, since it’s not bound by language.

The inner monologue is useful, but not required for intellectual thought. In fact, it can be a detriment. It’s hard to process things, when you don’t have the language for it. It is, however quite useful for presenting ideas. An inner monologue lets you practice what you will say, and how you will explain things to someone else. I’m autistic, so I often need to preprocess what I am about to say. My inner monologue lets me test if it’s “socially inappropriate” (aka batshit insane) before it comes out my mouth.