Ehh....that's a toughy, there are different reasons.
Sometimes that recommendation is given without context and reason.
Many folks feed a low protein(15-16%) layer then pile on the treats and wonder why their birds are not producing well. Higher protein is recommended to balance out those other 'foods'.
Some folks have multi-aged and gendered flocks, calcium is not great for non layers(controversial) so an 'all flock' type feed is recommended with oyster shell available to the layers to take as needed, they usually have higher protein(18-20%).
Feather and egg eating can indicate a need for more protein, especially animal protein. So more protein is recommended.
I use a 20% protein 'all flock' type feed, but I also like to give scratch grains daily,
the rough measuring maths work out to about 17-18% protein. The higher protein is good for growing and molting birds. You have to look at overall diet and nutrition and strike a balance that works for your flock.