Why should we estimate?

Why should we estimate? What are we considering when we do it? Yes, estimation accuracy is a matter of practice maintaining the same team constant. What and how can we estimate? We can do it all over hierarchy of requirements, since strategic levels until implementation ones. But essentially a good product planning should be done to slicing features maintaining product value and introducing complexity across the product development phases.

Requirements hierarchy, what should be written per level and by whom

(Berlin, Germany) Sometimes the most obvious Agile concepts are not so easy to be applied when we need to make a practical usage of them. Specially if we need to bring into a consistent and common understanding a product or portfolio vision for later implementation. My goal with this post is to share a very personal opinion about how and whom should fulfill those Agile requirements levels.