People on diets these days (i.e. almost everyone, at least at some point) try to "cut back" their fat. Modern nutrition guidelines specify that only a certain percentage of one's calories should be from fat. But in all premodern societies, including the Middle Ages, it was hard to get enough fat, which one actually needs (at least in moderation). One also needs it for cooking, especially in a pre-Teflon era.
The main medieval choices were olive oil and butter. There was no soybean oil, canola oil, or margarine. Lard was the third way (and some cooks still insist lard is best for pie crusts--I wouldn't know.)
Olive oil had been the Roman choice, as it had been for the Greeks. The "anointing with oil" events in the Bible were all examples of olive oil. It could also be used to burn for light. And athletes in antiquity, without today's "refreshing shower gel," got rid of the sweat by smearing themselves with olive oil, sprinkling on sand, and scraping it all off.
Olive trees grow well around the Mediterranean, providing both their fruit and oil. Southern Europe continued throughout the Middle Ages to rely on olive oil as the major source of fat.
Olive trees, however, do not grow well further north, although the oil certainly traveled on the trade routes. For northern Europe, butter (made from churning cream) was the chief source of fat in the Middle Ages. If salted and kept cool, butter will keep quite a while. In Scandinavia, butter might be stored for over a year.
When the Germanic people first came wandering into the Roman Empire, bringing their cows with them, the Romans reacted with disgust to this source of fat. But then the Germans weren't totally impressed with olive oil.
Butter works well for greasing the pan, mixing with flour for baking, and making dry food more palatable. But it doesn't work nearly as well for lamps, anointing, or scraping off the sweat.