This is mostly for the fighters.We all know the supplies the troops need to be trained(or the orcs needed for orc troops),especially in later chapters,and more when we use time boosts.
So,a wonder that will grand us a % reduction in training costs will be desirable?
thats why I like the buildings that give free troops. Temple of the Toads, Dwarven Bulwark, Flying Academy, Shrooms Shrine, Orc Nest, Various event buildings, orc strat building etc.