The security of the whole system depends on no one entity controlling >50% of the hash rate in the whole network (that is, the energy being used to mine). Furth

The security of the whole system depends on no one entity controlling >50% of the hash rate in the whole network (that is, the energy being used to mine). Further attackers with below 50% can impact the blockchain with a lower probability of success. More information better written at [0] and [1], but they'd be able to reverse their own transactions (and double spend), and also stop any other transactions being confirmed. So essentially these miners are making it more difficult for anybody to attack the blockchain, thus they're providing security. [0] http://bitcoin.stackexchange.com/questions/658/what-can-an-attacker-with-51-of-hash-power-do http://bitcoin.stackexchange.com/questions/658/what-can-an-a... [1] https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_...