I use it for reasoning. It’s a way to talk to myself without having to do so out loud, which I do a lot.
There is a segment of the population who, apparently, don’t have one. Even deaf people apparently have an inner monologue of hand signs visualized. But this segment just lacks one entirely. I don’t understand how they think, how they come to a conclusion. Things just pop into my mind, when I take my mind away from other matters and let my subconscious bake on an item… is this the way they think about everything? I don’t know.
Can you summon a song in your head? A melody?
It’s that… But it’s your voice. And you employ it to think. It’s how I argue with myself and reason my way through a thing. I’m not sure how I’d get along without it, except every once in a while I get stuck on a problem, so I do something different. Often, the right solution to the thing I wanted to do will pop into my head. Then I need to work backwards how I got there. Both are useful, I prefer the information up front though.