The return air duct helps with even temperatures. For proper air exchange.
A higher percentage of the air is recirculated, instead of drawing the air immediately surrounding the fan/shroud, which is less efficient and less stable,so far as regulating temps.
To exchange all the air efficiently, and not just a portion of it, Sally's return air duct is beastmode! Plus I imagine the cooler stays upright better
Whether or not it's on the outside matters not.
The advantage is more interior space for more eggs!
Your AC wouldn't work properly without a return air duct either....
If you cannot draw air in, you want draw any out.