To be able to say "our map is 100x100km!"
The only games where it is worth it to have a huge map like that, is army simulators and RTS. Anything else could probably be better off with polish in some other place, rather than a huge map.
Games / game engines use units which correspond to size IRL. It's needed to keep scale consistent. The characters are usually around 1.8m tall for instance
One of the notorious examples in PS3 gen era that's now can't be purchased at all. It's a derpy offroad racing game in what looks like a procedurally generated world emptier than ash deserts in Morrowind.