I think the notion of specific rations for specific ages & types of birds comes from valuing economics more than nutrition.
Poultry Science has traditionally been more about economics ... how to produce poultry products as cheaply as possible ... so often was about finding the specific balance of nutrients the birds need in order to perform, and including just those nutrients in that balance and nothing more. You can get away with the slim margins on the balance of nutrients when the birds are going to die soon anyway.
But, as @gjensen noted, breeders are different. Breeders need to be fed well enough to be in great condition well before breeding season in order to to produce super nutrient-dense fertile eggs so the chicks that grow inside them will be healthy. Breeders are expected to live longer than meat birds or layers. By a lot. And so on.
We've always known that birds do better (health and performance) on extra nutrition (this is true for both broilers and layers), and we're starting to understand how the extra nutrition will produce more nutritious poultry products.
So, I thought I'd throw out there the notion that hobby farm breeders it MAY make sense ... financial sense, nutritional sense, common sense ... to use a breeder ration as a base feed for all the birds. I decided to consult a feed nutritionist/mixer/custom mill to see if that was possible, it was, we switched to that system, I can tell my birds are healthier for it, and it's less stressy for us humans.
That said, I do reserve the right to continue tweaking my feed if I feel I can do better.
And yes, we could save a few pennies by feeding layer rations to the layers, and chick starter to the chicks, etc. Except my breeders are layers in the off season and are also broody hatching and raising my chicks, and the broodies and chick are mixed in with both the breeders and layers. So, all the birds would have to be very honest about following my instructions about what feed to eat to make that work ...
For what it's worth, when I was chasing the "feed economy" ideal and had several types of feed going at once, I could NOT get anyone else here to understand how to put the right feed into the right feeders. And we're a smart bunch. It was maddening. It's so much saner here using breeder rations as our base feed. The birds are sure doing better.