What does she want? Why does she want it? What will happen if she doesn't get it?

In this scene, I believe Sally would like to have the audience in the palm of her hand. The audience watching her on stage in the film and the audience watching the actual film.  I think she wants this for a few reasons. The Emcee gave her such a big a big introduction I think she feels she has to deliver amazing performance.

I also think she wants to prove to people that she is amazing and works hard for her money. 

If she doesn't get the audience on her side, they probably won't come back to see her perform again, which means she won't get any more money to live.