- Why is it important to make sure that miners spend money (on electricity) while mining?
This is how the Incentive works. The costs ensure the miner follow the rules of mining. If they don’t spend money, they don’t have any restrict, this equals they could break the rules without any punishment, they could do anything they want.
- Why would it be very difficult for someone to go back change a previously added block?
Because when man go back change the previously block, every following block has to change too, meanwhile other miners are adding new blocks. It demands super hashpower which is almost impossible.
- How does the network regulate mining difficulty?
By using target, only the hash number is less than the target, that block is accepted by the network. The more miners, the lower target, the more difficult to guess the nonce.