Simplified operations, faster reaction time, eliminating human resistance for obeying killing orders. See "War Games" [0] for a hypothetical exploration of the concept.
> a chatbot.
Some claim it's self-aware. Some say it called for airstrikes. Some say it gave a hit list for them. It might be a glorified Markov-chain, and I don't use it, but there's a hoard of people who follows it like it's the second Jesus, and believe what it emits.
> I can't see it happening.
Because, it already happened.
Turkey is claimed to use completely autonomous drones in a war [1].
South Korea has autonomous sentry guns which defend DMZ [2].
[0]: https://www.imdb.com/title/tt0086567/
[1]: https://www.popularmechanics.com/military/weapons/a36559508/...