Been reading the manual, was confused about the exchange. Why would you want to place cubes back unto the field instead of placing in the vault.
Maybe the vault is full?
You could use the exchange to pick up cubes if your floor pickup breaks and the portal is out of cubes. It is also possible to design for use of the exchange if that is your strategy.
Kind of a little strategy, but if you don’t have an intake to get the cube in a certain part of your robot or such, you could push the cube into the exchange and have a human player put it into your robot via the return. I’m not sure how many teams are going to use this, but that’s one reason why someone may use it. 
Good luck this season!