Honestly, no idea.

I could imagine a Darwin Machine being a "pure brain", as you put it. While we have emotions because our brains built a pure brain atop an existing and messy infrastructure.

Or, emotions could just be the subjective experience of thoughts competing.

Calvin goes deeper into things like this in the book, but I suspect emotions help intelligence to some extent insofar as they provide environmental change. It's good for the long term health of an ecosystem to shake things up so that nothing gets too stagnant.