"Rush" - what for are there double starting zones?

Maybe a dumb question, but I've tested this dungeon in hot seat and don't understand it. There are 4 starting/TD zones (2 per player). But we can deploy only in one particular zone and we can do a TD only in zone where oponent was deployed.
So, can anyone explain me why there are second starting zones for each player? Did i miss something?
So, can anyone explain me why there are second starting zones for each player? Did i miss something?