Well, yes, like for code comments, the most important parts are the whys. You're also forgetting that in some situations the source code itself might be unread

Well, yes, like for code comments, the most important parts are the whys. You're also forgetting that in some situations the source code itself might be unreadable to the person : either because it's just not available (including the comments), or because the reader doesn't know the programming languages (with assembly programming being a bit of both).