Linked from
The 13 pages that link to Eggshell, each with the reason it gives.
Amniotic eggRelated: It limits water loss while allowing respiratory gases to pass.
IncubationRelated: The shell limits water loss while permitting gas exchange during incubation.
YolkRelated: The shell encloses the yolk and other contents while allowing gas exchange.
Leghorn chickenRelated: Leghorn hens characteristically lay eggs with white shells.