Agile means delivering very small increments to users at a frequent pace, allowing them to provide feedback early and often. When done correctly, this eliminates a significant amount of guesswork. However, if the client (or, indeed, your own team or management) does not fully understand the process, things can go spectacularly wrong in many different ways.
We still need to explain what "agile" is. Ask yourself why do we "need" to deliver small increments at frequent pace? Isn't it to be able to react to changes?
It's main purpose is to react to changes, and make the whole process sustainable. It's literally one of the 4 main lines of the manifesto and the second of the 12 principles, and included in the meaning of the term "agile" itself, while guesswork isn't mentioned. But I can see how trying to predict what the markets will look like in 6 months involves a lot of guesswork, which is eliminated. I hope you can see that the only reason why guesswork is tricky is that there are a lot of changes in the meantime, so the ability to react to them is the deeper, actual main purpose.
