As long as bicycles continue to be made in a wide variety of wheel sizes, gear inches or the European "development" (rollout) are the only ways to make direct comparisons between 2 bicycles with different wheels. Both these tell you the whole story, and 80" is 80" for 20" folding bike as well as a 29r mtn or hybrid.

Other than making bike to bike comparisons, it's all an artificial construct, and we could speak in any other measure we want, ie the gear ratio, crank turns per mile or what have you.

I commute on a 26" mrn bike platform, and ride 700c on the road. Gear inches tell me what I'll need to maintain certain speeds or climb various hills.