You don't have to I guess, but for Buckeye breeders to have nice sized birds, that's what they like to start them on.
If you are raising show birds you need to feed them a regimen that gets them to peak condition at show time. A higher protein diet from hatch will help them achieve the size that show birds need. There are a few tricks in feeding them to help win that ribbon other than protein.
If you are raising meat birds you need to feed them a regimen that gets the most meat on the body at butcher time, whatever age you butcher. What feeding regimen is best will depend on what breeds or varieties you are raising and when you want to butcher.
If you are raising chicks that will be egg layers you need to tailor your feeding regimen toward that. As LG said you can get a lot of different opinions on what is best for that.
If you are raising them for pets or some combination of these you might find a different regimen best. A lot of this depends on what your actual goals are and how hard you want to work toward them. As you can see from the earlier comments some people have put a lot of thought and work into trying to figure out what works best for them. It's not just feeding regimen but what breeds, types, or crosses to raise, whether they breed them themselves or buy chicks, whether they pasture or not, just a lot of variables.
I'm a lot more casual about this than many. My main goal is that I like to play around with genetics. I eat as many females as I do males since half if what I hatch are females. Some people sell their pullets for a pretty nice price instead of eating them. We all do these things differently.
Since there are only two of us size isn’t all that important, I can get two meals for us out of a fairly small hen or pullet, the second meal is soup. A larger male just means I get leftovers for a lunch or two. I do not fry, grill, or roast them but use slower methods so I don’t have to butcher by a certain age. Mine forage for a fair amount of their food so it doesn’t cost me as much as it would if I were buying everything they eat. I still spend a lot of feed.
To me the eggs are more of a side benefit, a very nice side benefit. We get a lot more eggs than we can eat and I get enough hatching eggs to fill the incubator when I need them. The excess are given away to relatives, friends, or a food bank. I do want hens that lay a lot of eggs but the eggs don’t need to be all that large. They mainly need to hatch well.
I start my chicks off with a 20% protein Starter for four weeks or until that bag runs out to get them off to a good start. Then I switch to a Grower. That’s what they all eat from then on with oyster shell on the side for the ones laying eggs unless I have young chicks. Then they all eat Starter until the chicks switch to Grower. They get kitchen scraps and garden wastes in season, but these are inconsistent, consider them more occasional treats instead of part of their steady diet as type and volume change a lot.
There is a lot of great information in this thread and some opinions. In my opinion you have a fairly wide latitude in how you can feed them for any for these goals. Different things can work well. How much effort people want to put into studying this and putting it into practice is up to them. I’m always in favor of people studying, thinking, and trying things to see what works best for them.