Why do you think Miss Havisham manipulates and misleads Pip into thinking she's his secret benefactor? What could she get from this behavior?

I think that Miss Havisham is a lonely old lady. Pip showing up gives her a sense of control over other people's fortunes. This makes the old lady feel alive. Miss Havisham also has a lot of rejection issues herself spilling out (the creepy wedding room) that will come into play.