As miners are paid to help secure blockchain, the more miners are working, the harder an individual can gain access to one particular block. Miners ensure that all bitcoin transactions are valid whether to track down or block a transaction from happening.
Mining makes it incredibly more difficult for anyone to reverse a transaction that has already been made because, in order to so, that person must rewrite all the blocks that follow the desired transaction. It also can prevent fraud by making it impossible for one to draw back from their own past transactions.
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