All the explanations I heard for the Gas usage, could imho also be dealt with in Ether directly. What is it that Gas does that couldn't be done with Ether directly?
It decouples the cost from the floating exchange rate of Ethereum to other currencies.
The gas cost is fixed and measures the computational workload. This does not change with the rising and falling value of Ether, much like the amount of fuel your car burns to get from point A to point B is indifferent to the price at the fuel point.
The gasPrice adjusts the cost as measured in funds, so if the price of ether happens to increase, then the price of gas can correspondingly decrease. Additionally, it solves for network congestion by allowing users to bid for priority.
Hope it helps.
If EVM opcodes were priced in units of ETH, rather than units of gas, then there'd be no way to decouple the cost of a transaction from the USD value of ETH at a given point in time.
Without this decoupling, if the price of ETH/USD doubled, real the cost of performing a transaction would also double.
By implementing an abstraction - i.e. by pricing opcodes in gas (which is a unit of work) - and by allowing a market to form around the price of a unit of gas, you can protect the cost of running a transaction from fluctuations in the USD price of ETH.
Relevant: What is meant by the term "gas"?