Why do we think they included Exterminatus as an option?

I appreciate it's a bit of an odd question but I would be interested to know what everyone else thinks. I know they said we will get more actual game information in the next year so I don't want to go over the same points that have been talked about the campaign layers. But hearing them "advertise" that point again with its own mini question made me ask myself: "Why would I blow up a planet?"

Logic-wise the idea is to do it when things get too bad and it probably costs a lot. But since the ancient times of the first total war games, the reality of the situation has been that if something starts going bad, players usually reload their save. I am not saying they should or shouldn't do it, it's just a fact that most people, on most video games out there do this.

With the lens of the current total war games, it is hard to imagine the AI on a "traditional" campaign map getting so out of hand that you'd just rather blow it up. And I would at least hope that calling it, lessens or potentially removes the reward you would get for beating the flashpoint normally.

Do they intend for us to show up on a planet, find it completely crawling with enemies and decide this is not worth the hassle and blow it up? Or do they intend for us to "organically" lose and decide to cut our losses with it?

Also, a bit off topic, but I had no idea other factions had planet-destroying measures outside of the Black Fortress thing showed on BFGA2.