You know what's just as important as what you're working on but almost never gets talked about? What you're not doing and why you're not doing it. And that's a problem. There are a bunch of reasons why describing what you're not doing is just as important as describing what you are.
First on that list is that it helps you decide if a task fits or not. Let's say you're working on a function that takes in an image and calculates a histogram. Simple enough. You do your RFC, you pass around the API and everyone is happy and you begin to implement it. You're almost done, and someone says "We've got this other project that uses compressed images. Why aren't you able to handle that?" You know that wasn't part of the plan, but not everyone knows what's in your head. If your docs explicitly said "This will not handle compressed images then either requirements and schedule would have been adjusted or someone would have built the shared converter. At the very least you wouldn't have been blindsided by the request.
Along those lines, it helps to clearly define domains and separation of concerns. If you know and share what you're not doing, it's much easier to see the gaps between components and fill them if needed. Right now I'm working on a shared Authentication model for users and services that spans clusters. I'm not working on Authorization. It's a very closely related item, and the same group of people will probably be at the core of that too, but it's a very different problem. And our docs are explicit about that. Yes, we know it's coming and we know how it will fit in, but we're not working on it now.
Another way non-goals can help is when you're defining the actual task list. You have a set of goals, and you have an idea of what the end-state looks like, but something about it isn't quite right. And you keep adding new requirements to try and bend the result into what you're looking for. You end up with some really specific rules. Something like "schedule the meeting before 3:00 unless it's a Friday, then do it before Noon, but don't schedule it for early on Monday, or around lunch". That might be easier said as "Don't schedule the meeting when people are catching up on old work or will need to leave before we're done, even if it runs long" That's much simple and covers the real requirement.
So next time you're defining what you're doing, regardless of the scope, make sure you include the things you aren't doing and don't want to happen as well.