ERDs, PRDs, RFCs. Regardless of what you call it, a lot of what these documents do is define what you're doing. Often in excruciating detail. And that detail can be important, for lots of reasons.
First, it forces you to think through what you intend to do. It forces you to think of corner cases and happy paths, and ways things can go wrong. It forces you to understand what you're doing well enough to describe it to others.
Second, it lets others know what you're doing. This lets them plan for or around it. It lets the group identify duplication. It lets others provide their experience so you can avoid past mistakes.
Third, it lets others know what you're not doing. Again, this lets them plan for or around it. They know what they'll have to do themselves, or at least find some other solution. And if you've added reasons why you're not doing it you might even help them avoid needing that thing too.
Fourth, it defines done. If you write down all the things you're going to do you have a checklist you can refer to to know when you've done them all. It's always good to know when you're done.
But what all of that doesn't do is say WHY you're doing it. And that's at least as important, if not more so. What problem are you solving? Who does it benefit? How does it benefit them? What's going to be better for the user/customer because of what you're doing?
There are two big reasons that why is important. First is that we can't do everything, or at least we can't do everything right now. That's why we have priority and severity discussions. Since we have to, at least to a certain extent, serialize things, the order of operations is important. The why is the input to the prioritization.
The second reason goes back to defining done. If you haven't defined the goal, the problem to be solved or the benefit to be enabled, you've just written a task list. You can complete the task list, but that doesn't mean you've solved the problem or enabled the benefit. If you have defined why then you've defined done in a much more impactful way.
And that's really why we're here. To have an impact.