I do that when I'm alone. I think it's because when I speak my thoughts it seems to make them more clear. There could be an element of loneliness-alleviation in it too. It might fool the brain into thinking it's got company.
Yes one downside is that some people will think you weird if they notice you doing it. I keep getting caught out, and I'm thinking of getting an earpiece so that people will think I'm having an audiocall with somebody. It's a shame people judge talking to yourself like that, but even if they didn't I think I'd still feel embarrassed if people heard me, though I don't myself judge people for talking to themselves. That would be irrational, because I talk to myself and there's nothing much wrong with me. It's not as if I seriously think I'm talking to a real other person, that would be delusional. Mind you, the majority of the human race has traditionally talked to spirits and deities, and some of that was very likely a misapprehension. I guess the difference is that there's safety in number - if everybody's delusional about the same thing, it doesn't count as being weird or mad, although objectively perhaps it ought to.
Another possible downside is that if you do it a lot, you might get used to doing all the talking, and Aspies are notorious for hogging conversations as it is, without doing anything to make it worse. I've noticed that the conversations I have with others are more reciprocal if the other person interrupts me when I'm going on too long, whereas if I talk to a "good listener," it's hard for me to avoid talking too much. There's a saying about talking to yourself - "you'll never find a better listener." I think it's very true, so I think it may be easy to get spoiled if it's overindulged.