Miles per gallon is another example. It's a useful number.

Income through taxes is roughly a percentage of GDP.

You could also just compare interest spending vs budget, and lots of people do. Spending on interest is roughly $1T out of a total $7T with income of $5.23T

$1T of an incoming $5.23T is pretty concerning. Especially given projections that the $1T is likely to go up significantly over the next decade.

Its good as a rough score to do relative comparisons between countries (and actually Debt/GDP is useful in that sense too), but as an absolute amount it doesn't mean all that much.

What matters is how much the debt servicing costs versus government revenues. Also how much that debt is growing (deficit) and/or what it would cost to reduce it.

But there's not much of a consensus around what is too much or too little.

I suppose 100% Debt/GDP is a good arbitrary number to raise the alarm, but it doesn't mean much on its own.

This ratio isn't just better because the units match, you don't have to adjust the units for growth and inflation.