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.
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.
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