[elmomalmo]

An agile process isn't just there to manage the developers, it's there to engage and manage the customer as well.

In my experience, customers rarely know exactly what they want out of a software project, large or small. Complicated documents, produced up front get ignored because of TL;DR and projects start on faulty specifications that describe a solution to illusory requirements. So, inevitably, there will be a disparity between documented requirements and the desired outcome.

Iterative delivery - a key feature of agile as I have experienced it - gives the customer multiple chances to try-out and feedback on working software along the way. Furthermore, by making the customer part of the solution early they are as instrumental as the implementers in ensuring the validity of the software and - therefore - the success of the project.

Incidentally, I think this is why many customers (internal and external) are suspicious of and hesitant to engage in a truly agile process. That is, they would become equally liable for the success of the project because of the decisions they are required to make. They are part of the solution and therefore potentially party to the blame for any failures. It's much safer for them to be more likely to fail and avoid the blame than it is to risk complicity in a failure, however less likely that failure might be as a result of the process.

What agile does that the author missed (or at least declined to mention) is that it gives a name to and formalises an approach that attempts to manage the customer as well as the implementers. It compensates for the fact that customers can't always be expected to fully understand their own requirements and for the inevitability that they change their minds.