I don't know specifics of ETH, but if it's anything like other cryptocurrencies, it uses a blockchain as a ledger for all transactions. adding to the chain blocks with proposed transactions has significant computational cost, so miners are allowed, by design, to add a payment to themselves to the ledger, out of thin air (as in, no payer). nevertheless, there are only so many transactions that can fit in a block, and if there are too many proponents of transactions, or if the costs of processing them are more than the built-in mining payment, miners can select transactions that offer higher payments for processing, in a sort of auction for priority in processing. given free competition among miners, I suppose the theory is that the transaction processing fee will narrow in to a fair market price, while making room for priority pricing