Hmm ... I just read about it this week in "Storey's Guide to Raising Chickens". Took me awhile to find the info again, on p. 115.
Basically, you don't want your hens to grow too fast and start laying too soon, before their bodies are ready. The risks are fewer and smaller eggs, and a chance of "prolapse" (don't ask me what it is).
So they recommend, at 8 weeks, to gradually add oats until you achieve a balance of 50/50 and continue to feed free choice. This measure reduces protein and slows growth. At 18 weeks, gradually reduce oats and switch to lay formula. Once they start laying eggs, switch completely to lay formula.
Now ... all this is from the book. I'm a newbie myself and my chicks are only 4 weeks old. I haven't heard anything about this on BYC. Anyone cares to comment on this method?