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In the movie The Piano, Ada is a mute, who likes to play the piano. When she's married off to Stewart, she insist on taking the piano with her.

Baines, Stewart's friend, buys the Piano from him against her will. When Baines offers up the Piano to Ada in exchange for sexual favors, she's willing to indulge his every request.

Why was the Piano so important to Ada that she was willing to do anything to get it back?

Jane Campion, from The Independent (article by Quentin Curtis, October 1993):

She is sympathetically out of step with the rest of the world: too extreme, too purist, too wild - particularly in Victorian times - to be understood. She has too high motives, and the world will always disappoint her. The piano enables her to have a voice, becoming almost indistinguishable from herself. Their fates are linked. And because she doesn't speak, her intimacy with people becomes more instinctive and physical. Which takes us into the area that the film is exploring about eroticism and fetishism.

And on the relationship between Baines and Ada:

'He never forces her to do anything,' Campion says. 'He may start by pushing her, but he finds it important for her to come to him willingly. He wants to be loved: which is what separates him from a psychopath, who wouldn't believe that anybody could love him.'

Ada, a mute widow in Victorian times, is sold into a marriage in a foreign country and can only express herself through her piano. When her piano is taken away from her - against her will - she wants to get it back. She has nothing, except for her piano and her daughter. For sure, it seems like Baines exploits her, but as Campion notes, he loves her and she is actually willing.