As I said in the chat discussion:
It depends. Everyone views theatre differently, and everyone will see a show and think about it in a different way. It's hard to generalize theatre. In fact, I can't even generalize theatre.
Example: My best friend and I went to go see SA. We're both 15 years old, and she felt the show completely affected her and related to her and "spoke to her" and I thought it was a piece of crap. (Sorry..don't mean to be THAT harsh about it..)
My cousin thoroughly enjoyed Legally Blonde because it was mindless, entertaining fun. I thought I was watching a high school show.
My mother hated Talk Radio! while I thought it was brilliant.
The purpose you ask?
A definition of musical theatre I have learned is telling a story through song and dance. You're communicating with the audience when you are performing.
The affect on the audience, however, depends on the individual alone.
I mean, Denzel Washington? Gun to my head..of course.