> … the why is important > … a lot of developers only consider the how.
The why is someone else's job, so the developers should just ask them for a blurb to put in the PR for context, along with a note to the reviewer to ask that person for even more context if necessary.
I think there's a why with regard to the code. Why this "how" and not some other "how". (Why did you pick this algorithm, this pattern, this solution to the bigger business why.)