Ethereum is a proof-of-work chain, like Bitcoin, which means miners get ether as a reward for validating transactions. Miners decide if they’re willing to process the transaction for the fees offered. The reason to separate gas and ether is so that computation costs can remain stable even as the price of the cryptocurrency fluctuates with supply and demand in the market. So, for example, a transaction that costs 100 gas will always cost 100 gas, but the amount of ether the sender will have to pay the miners to process it will depend on the market price of ether.
