It's fairly likely that your favourite fantasy roleplaying game has, buried in the rules somewhere, an assumption that you're using six-mile hexes at the basis for overland travel. This assumption is almost as old as the genre, and is present even if you're using a ruleset that doesn't explicitly use hex maps. Ever wonder why D&D 5th Edition has paces of 18, 24 or 30 miles a day? That's 3, 4 or 5 hexes!
The idea that they're the 'ideal' scale goes back to at least 2009, and Steamtunnel's oft-cited In Praise of the 6 Mile Hex. And that idea has merit, it's certainly mathematically convenient. But we don't make maps for the convenience of mapmakers: we make them to describe territory. The 6-mile hex does a poor job of that, because it is too big.