First let’s define rancid. Dictionary definition is "adjective (of foods containing fat or oil) smelling or tasting unpleasant as a result of being old and stale". An example would be fish that is left out too long smelling “fishy”; it is actually going rancid.
We tend to forget that we are not machines and food is not just fuel to burn. The way we interact with our environment is quite complex and one of the most basic ways is through what we eat. If you consider the entirety of human existence, it has been only recently that we have had the ability to store and transport food to where it is needed. Before, we where dependent on the seasons and cycles of nature, and our bodies would adapt to those changes (like the bear whose diet changes and when high carbohydrate berries are available in the late summer and fall to allow him to accumulate fat for use during the winter).