Barns give a large annual bonus to the revenue from all livestock kept on the farm. The bonus, summed with bonuses from all other buildings, is displayed in the Buildings bonus cell on the annual report.
A barn (and a well) are also required for keeping cattle and elephants on the farm. One barn and well will suffice for all livestock of those types.
Like other buildings, tools, and livestock, barns can be sold again if the player needs cash. If all barns are sold or lost due to annual events, the currently owned livestock will not be affected. However, no new livestock that depend on barns can be purchased, until at least one barn is again present on the farm.
Events Affecting Barns
Fires destroy all barns and sheds.
Civil War involves a 75% risk that each building is destroyed.
Angry Mob destroys all the player-owned buildings.
WikiPedia entries on Barns and Granaries:
“Demonstration of techniques for constructing different types of barns(Balanta, bo or madinka) for seed saving in West Africa: earth, straw or both,” bede-asso.org, 2011: