BitCoin Miners, what are they for?

BitCoin miners secure the BitCoin system by extending the BitCoin hash chain and also publish new transactions to prevent double spending. The way BitCoins are designed, the only reasonable attack vector is to substitute one's own hash chain for the legitimate hash chain. Miners extend the hash chain, making it harder and harder to substitute, even with increasing computing power. By paying miners more than they could make by stealing BitCoins, it becomes impractical to attack the hash chain.

Every miner does all the computing work they're doing based on the current hash chain. And the hash chain with the most computing work wins. So an attacker has to do as much work as all the BitCoin miners combined to attack the chain. The BitCoin miners are the good guys who ensure that bad guys can't undo BitCoin exchanges by outcompeting them.

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They're basically password crackers, but attempting to assemble valid and accepted Bitcoins instead. They have an extremely low rate of success.