What is the benefit of plum vinegar?

Not for health: Various kinds of vinegar are promoted as being good for everything. This alone should tell you that these promotions are sucker stuff. If you like plum vinegar, for example on your salad, that's fine. If it made you healthier, there would be barrels in the NIH cafeteria for the best-informed medical scientists in the world. Maintain your health through sensible diet, fitness & meds if needed.