While I was doing research on what to feed a parrot, when my first bird, Kiki, the Senegal, showed up, I read about pellets as part of the information that was available.
I learned that before pellets most people were feeding their parrots an all seed diet and the parrots all died well before they should have from heart attacks, fatty liver disease, kidney failure and even diabetes. This was attributed to the all seed diet that they were given. Then the feed manufactures got the wonderful idea of making pellets for the birds, their idea was that the root of the problem with an all seed diet was that the birds were picking out only the seeds that they liked and since these were the ones that were highest in protein and fat the way to remedy this problem was to grind up the seeds and make a pellet so they could not pick out their favorite seeds.
While this may have helped to some degree it also introduced a mild but chronic form of dehydration into the picture and still did little to prolong the lives of the birds and although the manufactures touted these pellets as a complete diet, it was still an all seed diet and the birds kept on dying from the same diseases.
There has been a lot of improvements over the years and there are many pelleted foods available that are not made from seeds alone and manufacturers still claim that their pellets are a complete diet for birds, but the science from sources other than the manufacturers says that these claims are simply not true. Except for some manufacturers, there is not reputable source that I am aware of that recommends feeding more than 30% of a birds daily diet as pellets.
The majority recommend fresh raw fruits and vegetables, some whole grains and some thoroughly cooked legumes and no more than 30% pelleted foods.