Dry seeds do not provide a balanced diet for any bird species, even finches and parakeets which eat a seed-heavy diet in the wild. Most parrots need a varied diet that is heavy in vegetables, with grains, fruits, and nuts/seeds offered in smaller amounts. To help provide a balanced diet to captive birds, you need to provide the right amount of protein along with important vitamins and minerals, especially vitamin D and calcium for bone health and egg production (if female).
It can be very complicated to find a good diet of fresh foods that is both available in your areas and acceptable to your bird. Many birds are picky eaters and will not eat everything that is offered.
Pellets are a nice solution to this problem because they are a uniform processed food, typically grain-based, that is supplemented to provide balance nutrition in a simple, shelf-stable package. Easy to feed and superior to a seed-only diet, but pretty bland and highly processed. Be sure to do your research to find a good pellet from a trusted company. Keep your pellets fresh by freezing the unused portion until you are ready to use it or storing them in an airtight container.
Alternatively, you could feed fresh foods and use a powdered or liquid vitamin supplement to fill in nutritional gaps, like vitamin D.
In my opinion, the best diet has a mix of all three - pellets are the foundation of the diet, with fresh veggies or chop making up the majority, if possible, and seeds/nuts being offered as treats or foraging rewards, in much smaller amounts.
In reality, many people struggle to find that perfect balance, since many birds may not want to eat pellets or refuse to try new foods at all. Seeds are yummy, but they are basically junk food. Most birds will eat it, but seed is high in fat, low on vitamins. You can live on hamburgers and fries for every meal for months or years, but your health WILL suffer for it. The same is true for birds on all seed diets.