That was the main one, and also in case you ever ended up with car trouble you had a record of every place you had filled up and every time you had the car serviced. It was also financial: you could tell how much you were spending on gas, whether gas prices were going up or down, which stations had the cheapest gas, etc.

Nowadays a lot of this is recoverable from your credit card statements (though again, I don't know anybody who actually does this), but in the 80s you would usually pay cash for gas.