I think it is just to give the users an idea of where the game area is. In the BoB Map it is so huge that you normally won't be using most of the map. When I start a multiplayer mission that I have set a battle area in, the game's map focuses on that area.
If you keep the sectors to 10000 meters, then each square is 10km, which makes it easier to navigate. But it's up to you.
I don't know if the game imposes any restrictions or penalties based on the battle area..
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