risking the start of a food thread, foods with butter added to them are softer... bread and water makes a corse bread. add some butter, and you get soft bread... (and sugar and butter and you get a cake)

add just sugar to flour and water, and the bread is sweet, but not as soft as when you add butter (or other fat) most "sweet breads" (not sweetbreads!) have very little sugar or honey but they do have a lot of fat.

as it does for bread, it also does for liquids... soups or gravies with added butter are smoother.

leathers are tanned with tannic and other acids, and some of them are fatty acids..maybe some of these have buttery names (and help contribute to a softer feel..)