Percentages can be good for quick mental comparison between choices, or straight calculations.
For example:
Planned groundspeed 130 kt.
Say time for one leg of a nav is 33 minutes, do groundspeed check from two pinpoints and actually getting 115 kt.
That's 15 kts slower than planned, close enough to 10% of planned g/s, so leg will take about 10% longer, or 3.3 min, so you can easily revise ETA and fuel burn.
It's not perfectly accurate, but can give you good estimates which you can refine when you get time.