If someone complains that our system did something weird, sometimes it’s a mistake we can just fix, but sometimes it’s a non-obvious consequence of some requirement (which I might not have been aware of) that we have to explain to our consumer. It also helps a lot to tell whether it has been like that for a week or five years.

I can’t even imaging having a spec that fully answers anything like this; Microsoft tried for that level of detail but I found they couldn’t keep it up to date.

Because you'd want to know why code does what it does.

If a bug pops up, you'd want to know why some code exists and if the original premise of the code is as it should be or needs revision.

I think it's very rare that a developer receives all that information up front in a team. Eg. Old codebase, lost knowledge or people leaving.