Maps exist to define ownership. Everything else is tertiary to that. Lewis and Clarke weren't "blazing a trail" for the pioneers. They were measuring the land that the US government had just purchased.
Sure. But the USGS doesn't publish precise up to date topological maps of every square inch of the United States for the purpose of guiding backpackers with compasses. They do it to divide up political boundaries, delimit resource rights, and guide commercial activity. This goes for just about any other large scale mapping effort (on earth).
