I guess because if you see a play being performed you might get the play more. For example if you don't understand how the play is being set than you get a better experience on how its being set or done. now if you were reading about the play i might give you info but some people like to see visual act then.
Please let me know if this helped. I'm not much of a explainer.
It makes a difference because reading it in your head can sound different. Especially in the English language, words can mean different things depending on the stress on each word. If you hear a play performed out loud, there's not a lot of ways the text can be misconstrued, as it's being acted out and said in the exact way the play-write intended it to be.
