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From Wiskipedia

Correct. The computation is catierng a highly valuable record of bitcoin transactions that gives the currency its value . Without this competitive computational effort, the transaction record would be easy to forge or manipulate.Think of it as building the sewer system underneath any major city. The cost is absolutely enormous, as is the energy expenditure. But the result is a very useful public utility that -in the case of sewers has an immeasurable impact on human health.I predict that Bitcoin will eventually have a similar effect to the monetary system as sewers have had on sanitation. Whereas sewers help to carry waste away, preventing the spread of disease, Bitcoin enables lower cost financial transactions and the removal of intermediary layers (credit card companies and financial institutions) that stand in the way of free and open commerce.How much energy could we all save by taking down Visa?