What actually is a proven dog? Proven in which way? How? Let's take an ideal example.
I get a dog (Wade for example :D). I am a working breed fun, so be aware that I am getting working breed here:)) He has some pedigree, some parents. This is not making him proven. Just work promising.
If you do not know if the dog really inherited qualities out of his parents, you cannot be sure that there is something to pass on.
I watch how the dog behaves in different situations and during training foundations. Let's say that this dog has got a nice food and toy/prey drive. He has will to please, is naturally social, does not react on sounds and has stable nerves. About one - two years of age I pass some basic exams and I see, that there is also some working potential. During that time I do health checks and I see, that the dogs results are ok. So I add some work load and go for competitions. Go for class two, ideally top class three.
At that time I have healthy dog proven in some work - this means that at that time the dog is becoming also promising for breeding. So I take him to get a breeding licence. And make some first breedings.
Then I watch this dogs puppies out of few litters - and if those are fine, I can say that I have a proven dog. Proven in work as a first step. And then proven also in breeding.
Of course this described process is the ideal one. It can happen that during this process I find out, that the dog does not have necessary qualities to prove his paper abilities - potential based on his pedigree. In this case, this dog is not promising for breeding and therefore should not be used.
This means, that the dog is becoming promising for breeding after he is promising for work and then proven in work. And after this all, when you can check his offspring, you can say that he is also proven in breeding.
So what happens when you use in breeding dogs, who are not proven in work? You get very uncertain results. If you do not know if the dog really inherited qualities out of his parents, you cannot be sure that there is something to pass on:)