The best laid plans
As your plans get more detailed, it's also more and more likely that they won't work exactly as you described them.
Certainly, it's worth visualizing the thing you're working to build. When it works, what's it going to be like?
Even more important, though, is being able to describe what you're going to do when the plan doesn't work. Because it won't. Not the way you expect, certainly.
Things will break, be late, miss the spec. People will let you down, surprise you or change their minds. Sales won't get made, promises will be broken, formulas will change.
All part of the plan that includes the fact that plans almost never come true.