Why is there a distinction between gas and ether?

I'm learning more about how etheruem works and I'm confused why theres a distinction between gas and ether. From my understanding when a transaction is sent it spends a certain amount of gas which is denominated in ether. The price of gas to ether is a floating value that depends on demand for that block and you spend a certain amount of gas at that price of that block to send a transaction. Why is there a distinction for gas at all? Why not just spend a certain amount of ether per transaction instead of converting from ether to gas then converting back to ether? This seems very arbitrary and confusing so I'm wondering why ethereum has gas at all instead of just using straight ether.