How is such a definition more useful? It's certainly not more useful to me. If someone tried to sell me such a quantity of land, the first thing I'd ask would be what the actual size is in standard units. A universal, precisely defined system of units is superior because it works in any context, you just need to know the system.

Expressing distance by travel time is a similarly stupid concept. I can tell someone something is 15 minutes by car, but here I'm expressing time, not distance. If I tell someone something is 15 meters away I'm expressing distance. The separate concepts of time and distance allow me to precisely express whatever I need to.