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.
It's a tool that can be very helpful for describing the logistics of travel.
Depending on where you are and the context of the conversation, knowing that something is fifteen miles away might tell you shockingly little about the experience of getting there. There are places where that distance in one direction might take hours, but in another direction minutes. As you might imagine, this can be important to know.
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.