Writers can show a whole world, describe conflict, and trick the reader just by fine-tuning a character's voice to make it sing. How someone chooses to express their emotions, surroundings, and the situations they get into can convey way more to the reader in a more engaging way than simply stating it.
Nondialogue voice or your character's inner voice is usually done in a way that aligns with how the character speaks with the rest of the characters in the story. But sometimes an author wants to show that a character has extra depth or perhaps isn't who they appear to be to the reader or other characters in the story.
Using character voice as exposition may seem like a simple technique, but more often than not, writers do this in a heavy-handed way. This leaves their dialogue stilted, dry, and an obvious story function that drains their prose. Blending story exposition and important story information allow you as the writer to keep your story engaging while feeding your reader what they need to know to stay grounded in your story and characters.