From a reader:
Trans fats are a synthetic substitute for the saturated animal (and tropical) fats that were commonly used as shortening and for frying. They became popular because of their supposed health benefits, and a growing distaste for animal products – not for their taste. The big switchover occured in the 1980′s after campaigns by people like Ralph Nader’s CSPI.
(Interestingly, the CSPI now claim they had no idea McDonalds would use partially-hydrogenated oil when they stopped cooking their fries in beef tallow, even though it’s well known that most unadulterated vegetable oils are unsuitable for high-temperature frying.)
After hydrogenation, trans fats are heavily processed to remove their unpleasant taste and smell. Some claim this very /absence/ of flavor makes them a better shortening than lard. However, McDonalds now add artificial beef flavor to their fries, and butter continues to taste better than margerine.