By reducing the size of the cell to back within the natural range spectrum we fit more naturally the natural flora of plants not hybridized by man, which is the majority in Nature and thus increase the diet, making it more balanced for overall colony health. This can be seen in a greater range of pollen colors brought into and deposited within our colonies. It also makes for more range in the collection of nectar also for increased health benefits.
By reducing the size of the cell back to within the natural range our bees can breed back and forth with feral bees and gain greater variability for expanded characteristics for selection, and where one has great selection, you can breed more for what one wants.
By putting all this together into a package then, we gain control for better diet for our bees, better disease control for our bees, by not having their bodies chewed upon, either externally or internally to vector in associated problems of viral, fungal or bacterial in nature, we gain reduced reproduction of parasites by natural restriction of their diet upon our bees, and the result is we don’t have to use various chemical treatments. I probably have missed someting here, I hope not in thinking about it. I hope it helps answer your question as to why we do it.