Theatre is a branch of the performing arts. Any performance may be considered theatre; however, as a performing art, theatre focuses almost exclusively on live performers creating a self-contained drama. A performance qualifies as dramatic by creating a representational illusion. By this broad definition; theatre had existed since the dawn of man, as a result of the human tendency for storytelling. Since its inception, theatre has come to take on many forms, utilizing speech, gesture, music, dance, writing, and spectacle, combining the other performing arts, often as well as the visual arts, into a single artistic form.
The word derives from the Ancient Greek theatron (θέατρον) meaning "a place for viewing. Modern Western theatre derives in large measure from Greek drama, from which it borrows technical terminology, classification into genres, and many of its themes, stock characters, and plot developments.

Imagine this... it is the age of the caveman. Human beings have begun to live in rough communities with a hierarchy and roles for specific sections of the community. Gender roles have been assigned, men to hunt, women to cook and care for the children. Language is still rudimentary, sounds that convey danger, excitement, gladness, etc., rather than recognisable words. One member has invented or stumbled upon a better way to hunt.

So, as the community gathers at the campfire at night, he steps into that circle of light and demonstrates what he has invented / discovered / learned.

That was theatre then and it still is theatre now. It demonstrates a problem and a solution. In some cases, the solution is not overtly presented but the audience is left to figure out the solution itself. That is because our ability to think in a language empowers us to run dialectic in our heads in order to arrive at a solution or several possible solutions, once the problem has been presented.

And the presentation of that problem is being done exactly as it was those thousands of years ago, by demonstration. Demonstrating a problem rather than narrating it resonates with human beings much greater than narrating it does, all because human beings are hard-wired, through evolution, to pay attention to theatre.

also theatre can show many different cultures and when looking at a performance done by Americans versus one done by the Chinese there very different and the difference gives you a depth on that country's culture - you can learn a lot about a country through their theatre

you could also say that because so much of drama is physical than you don't need to know what is being said to understand the message.