That is the thing. You rarely know what you need to build. With big upfront design your are speculating, the design is a house of cards. Agile amplifies learnin

That is the thing. You rarely know what you need to build. With big upfront design your are speculating, the design is a house of cards. Agile amplifies learning. You learn as you build because you get early and frequent feedback. Agile relies on having very capable product owners who understand what needs to built. Waterfall relies on design documents. Which will be all wrong anyway if you didn't have a capable product owner driving the vision of the product.