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.
