Something to consider.... parrots out in the wild evolved to eat the available foods that are around them. With the way fresh food production has been going, some things that we humans grow are no longer as nutritious as the original, unmodified plant. Most owners do not have access to what parrots would naturally eat out in the wild, so we have to compromise and try to "cover all bases", so to speak.
There's one parrot species, the pygmy parrot, that it's entire diet for the most part is comprised of moss/fungi. Due to this, no one has been successful at keeping these tiny little parrots alive within captivity. Another parrot, the red bellied macaw, feeds almost exclusively on mauritia palm fruits and nuts. Most RBM owners do not have access to that kind of fruit, yet these birds thrive on what could be considered a very poor diet for any other species within captivity. Sunflower seeds, peanuts and foods high in vitamin A!
For the most part, a varied diet within captivity is to try and cover what they wouldn't be getting in the wild. Sometimes, even then, it's not enough. Even birds on varied diets can still end up with nutritional deficiencies. Calcium and vitamin A are usually two common deficiencies seen within captive parrots.... Calcium often due to a lack of vitamin D (i.e. sunlight!) and vitamin A due to a lack of vitamin A foods.
