But they sure help detecting problems early. And that's extremely valuable.
For instance, iterative development with deliveries early in the project will test the technical feasibility and the development speed. But perhaps most important, we will get feedback on the functionality from the clients, to verify if we have understood their problem correctly.
Failing in any of these areas will lead to project failure. And it is better to discover this when 20% of the budget is used than when 90% of the budget is used. First, you reduce your losses. And even better, you have a fair chance of fixing the project and turning it into success.
Many agile projects succeed not because they are more productive, but because they discover problems in early iteations and then reduce the requirements.
Unit testing is another practice that makes you aware of problems early. This makes it cheaper to fix problems, but it also gives confidence, so you dear restructure code when necessary.
Another agile practice is close communication with business experts, preferably face to face. The non-agile alternative is written requirements. The problem is that developers misunderstand what the business experts want. That's inevitable. What we need, are mechanisms to discover misunderstandings. That mechanism is called feedback. The business expert explain what they want, and the developer explain how they understood that to the business expert, which confirms that this was correct. That's easier to do face to face than with written documents.