In the diagram above shifts in the supply curve between S2, S3 and S4 will only result in the price changing between the acceptable price band. If a supply shock causes the supply curve to shift to the right to S5 then the buffer stock authority will intervene and purchase the surplus Q4-Q5 thus preventing the market clearing by itself through a lowering of the equilibrium market price to P1. If the supply curve shifted to the left then the buffer stock authority would release stocks equal to Q1-Q2 on to the market thus preventing the price rising to P4. It can be destroyed It can be sold to other countries It can be given as overseas assistance. Each option has a number of costs associated with each.
Storage is expensive and involves an opportunity costs of the storage facilities.
Destroying surpluses especially if the surplus is a food is morally questionable in a world devastated by poverty and hunger. Selling to other countries at low prices or dumping can undermine domestic producers in the countries where the goods are sold. Giving the food as aid could, it is argued, lead to a dependency culture.