If that sweet feed is basically scratch mix with some molasses added, then it's not a complete diet for chickens. They'll love it, just like kids love cake, but it doesn't contain all the nutrients a body needs.
All protein is not the same. Protein is built from a variety of different amino acids. Muscle tissue in animals and various parts of plants don't utilize or contain the same amounts of the various amino acids. This comes up when people talk about amino acid profiles in foods, balanced proteins, complete proteins, etc.
Two foods can contain the same amount of protein by % and not contain the same amount of various amino acids. When you are feeding a person or a chicken, they need enough of all the various amino acids to build and maintain their bodies, as well as create an egg. They can't substitute extra amino acids of one type that are in a food for the types that are missing or in short supply in a food.
Some animals have different digestion systems and nutritional needs. What they eat and how they convert it in their bodies is different than chickens and people. Even the animals that eat sweet feed are eating other things, also.
The things in layer feed that aren't in sweet feed are the additional amino acids that grain is lacking (usually from soy, fish or meat, sometimes individual amino acids also added in) vitamins and minerals, especially higher levels of calcium.