Actors are required to know, well, everything. History, mathematics, language, science, art - if it is a subject in existence, then it is useful for an actor to know.
Actors are called upon to know how every person in every walk of life exists, and must be jack-of-all-trades kind of people. Whether an actor has to play a doctor, carpenter, ballet dancer, mythical creature, 1930's housewife, or president of the United States (hopefully not all at the same time), he has to have at least some knowledge of what these types of people do, how they do it, and how to recreate it in a realistic manner. Additionally, any skills in scenic design, building, painting, technical skills with lights, sound, and other effects, costume and make up design, or anything that might possibly aid somehow in creating a production make an actor a much more valuable asset to the theatre.
As an actor, every single thing learned in life is precious. Each second experienced is yet another token of information to store in the bank from which an actor may need to draw. It is impossible to know just what memory, occurrence, understanding, or observation will be useful in the future.