Back in the day, summer kitchens were most commonly old granaries, chicken coops or other types of out buildings that were renovated to suit the culinary needs of the farm or ranchers cook. In the spring, the old coal and wood stoves were moved out to the summer kitchen where the combination of the high temperatures of the season mixed with the humidity of the cook stove were kept separate from the main house. I imagine that it would have been desirable to leave the mess that comes with preparing and cooking away from the main house as well.
Even now, modern central air conditioning has a tough time competing with the challenges of indoor cooking during the summer months and the mess does not do anything for the decor. The idea of having a lovely little summer kitchen - with a little flower box full in the window - sounds like it's worth a second look to me.