The story I heard a long time ago is that Cories come from fast moving streams and are in the thousands. Apparently the fish are used to being in large groups so probably are programmed to react well in groups, the bigger the better. Tanks are, at best, bizarre to our fish so anything we can do to make tanks more like "home" is good.
Incidently you are probably aware but for those who are not; Cories are not garbage eaters; yes, they are foragers (generally in the substrate) but they seem to prefer live stuff or real food if they can get it but in a tank fish take what "God" offers 'cause it is not as if they have a choice, other than starving.