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In the mining of a Proof-of-Work consensus mechanism system, sometimes more than one block will be mined at a particular block height. In this instance, the network will choose the long running chain as valid, making it part of the ongoing blockchain. The blocks in the short chain will become stale blocks.
It is vital that the transactions contained in the blocks are applied properly to the ledger. Since cryptocurrencies like Bitcoin have monetary value, the ledger is vital for determining what is in each wallet. The payout of block rewards is the onset of this system.