It's a lot easier if everyone uses the same system to compare one bike with another.
Which is a higher gear, 32x19 or 34x20 ?
32x16 on a 26er or 32x18 on a 29er ?
It's not immediately obvious is it ?
Convert it to inches and you get 48.8" or 49.3"
52" or 51.6"
What could be simpler ?
For those who don't know, it's the distance travelled for each crank revolution. The higher the number, the further travelled, the higher the gear.