To figure out MPG you were getting.

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.

	▲	alex0015 42 minutes ago \| parent \| next [-]
		A friend of mine in high school (2012) was required to add to the notebook his parents kept in the car whenever he got gas. I wonder if his parents still do that kind of record keeping now that the car tells you its MPG, gas price historical data is easily accessible, and card purchases are tagged as "Gas" automatically in the banks that I use. I do actually audit each transaction in my card statements each month to look briefly at the categories of spending I use most. Gas is a category, but that number doesn't really change my behavior. It's just a data point.
	▲	snohobro an hour ago \| parent \| prev [-]
		I’ve been keeping a spreadsheet of all my fillups. Mostly because I was curious about my monthly spend on gas with it having gone up so much. But it’s been interesting to see the mpg over time.