I actually didn't know you could do that in those games, never tried it! I might do it with Stellaris now that you mention it!
But actually I was in a weird spot where I was craving playing some Sins but didn't have the mental capacity to actually play, so I thought it would be a lot of fun to open a huge map with high level AI (I initially wanted to try this with the Maelstrom mod in Rebellion, since it adds a lot more factions.) and just see what happens.
I think the capability of having these watchable AI matches would be a lot of fun for multiple people, and could even be a way to learn more about strategy or just how the AI works in general (yeah I know I could just watch actual human strategy videos, but that's not nearly as fun in my opinion ).