"Without test-first and refactoring, clients think they must assemble as many program requirements as they can afford to have written. This effort snarls all relative business priorities together, making Scope Control impossible. It obscures opportunities for simplification. Designing and implementing many features all at once is very hard, leading to our industry's reputation for very large failures. Putting tests in front of development's inner cycle permits an outer cycle of incremental feature growth. That relieves the Customer of the responsibility to predict the future and guess which complete set of features will maximize productivity."
.. Juergens answer:
Nice description. Sometimes I try to explain that to non technical people with the following picture: If you have only one chance to get your wishes on a list, it is like Christmas for a child. You make sure you get every little wish on that list and hope for the best. If you are sure that you can get your wishes on the list at any time. You will just put the most important ones there which come to your mind easily.