Here's why: Production houses have, of necessity, developed better, higher-tech cooling strategies -- ones that may be better than whatever you can dream up on short notice. Secondly, White Leghorns (commonly used for egg production) seem to have higher heat tolerance. And finally, backyard poultry keepers are sometimes drawn to the heavier, more fully-feathered breeds and these have more trouble with heat than lighter breeds.
• The ability to relocate the coop makes it more feasible to placate neighbors.
• A movable coop should avoid hurricane code requirements for fixed structures.
• If a storm threatens the coop can be moved into a garage.
• The chickens will frequently have new green territory to scratch up instead of ending up with a dirt-patch run that could become contaminated over time.
• The "tractor" will cultivate and fertilize parts of your yard.
• Chickens can be relocated to a neighbors for care if you need to travel.
• If you ever transfer your chickens to another owner, you can unload the coop too, a plus for both parties.