Here's why parents should let or allow their kids to fail.
1 It will teach them a lesson. Even as adults we fail and then we try again, hoping we won't make the same mistakes. Guide them, but don't point the correct thing to do.
2 They will learn what's good and what's bad. Of course, if it fails then they would see how that affected their progress/assignment. Take homework for example, if they don't do it then they will have to take up the consequences then they will understand it was a bad idea/decision not to do it.
3 They will learn to appreciate what they have. Sure there are other ways to teach them that, but since we are on failing why not? If they fail certain privileges will be taken away, that way next time they won't do their best, they'll know what will happen.
4 Teach them to appreciate the people that are there to help. If as parent tell your child to do his homework and he won't do it, then next time he won't have anyone there and it sure sounds like fail.