Why is Miss Havisham's mansion so important to Great Expectations?

Miss Havisham's decaying mansion, ironically named Satis House, is central to the novel as from it derive character development and thematic concerns.
Characterization
Miss Havisham
The daughter of an aristocrat who owned a thriving brewery, Miss Havisham and her fawning relatives represent what Dickens felt was a frivolous aristocracy. As the pampered daughter of a wealthy man who owned a brewery, Miss Havisham wanted for little, but her half-brother Arthur, born of a servant, was resentful of her and conspired with a clever man named Compeyson, who pretended to love her. When she was left at the altar by Compeyson, who had absconded with part of her fortune, the devastated and eccentric Miss Havisham stopped all the clocks in Satis House at twenty minutes until nine, and remained exactly as she was in her wedding gown, with one shoe not yet on, for the remainder of her life.
When she recovered some from her rejection at the altar, Miss Havisham adopted a pretty girl whom she named Estella. This child she has raised in the mansion to hate and malign men as a means of avenging herself upon males. Estella does, indeed, mature "without a heart," but her life is not happy and Miss Havisham later regrets what she has done, especially the personal harm brought to Pip. In fact, she dies in her mansion after begging Pip's forgiveness, for her decayed wedding dress catches fire and she is so severely burned that she expires.
Pip
After Pip's first visit to Satis House, his view of life is completely altered. For the first time, Pip feels inferior as the spoiled Estella remarks in his presence that he is common and coarse; repulsed by this "laboring boy," she does not wish to play any games with him. After he leaves, Estella brings him some food and drink at the gate as though he were a pauper, or even a dog.
When the "pale young gentleman" boxes with him according to the Marquis of Queensbury rules, Pip fights the only way he knows and knocks the boy down, winning the fight. Estella cheers for Pip, but makes fun of him when he cries another time. After he returns home, Pip feels ashamed that he is "common" and wishes he could become a gentleman.
When Pip receives news that he has "great expectations," he thinks that Miss Havisham is his benefactor, and he is elated to go to London where he can study and learn to be a gentleman. In so doing, Pip becomes a snob and feels embarrassed to be around the good Joe and Biddy. Thus, his exposure to the lives of Miss Havisham and Estella at Satis House have initiated Pip's becoming a snob. In fact, his sense of superiority keeps Pip from visiting Joe when he is near the forge. Instead, he stays at the Blue Boar.
Themes
The Worship of Wealth
The relatives of Miss Havisham do not like her, but they are flatterers and servile sycophants to her simply because they hope to inherit her property and wealth.
Uncle Pumblechook, who has continually berated little Pip, changes his attitude toward the boy once Pip goes to Satis House. When he learns that Pip has a wealthy benefactor, he extends congratulations to Pip, saying, "I wish you the joy of money." Also, he fawns before Pip, now calling him "friend" rather than "boy":

"to think that I may have been the humble instrument....to think that I....My dear young friend,...if you will allow me to call you so...."

At another time while Pip stays at the Blue Boar, he reads in the local paper that Uncle Pumblechook has taken credit for Pip's good fortune.
Class Distinction
Pip becomes a snob as a result of his experiences at Satis House. Once he begins his life as a gentleman, Pip is embarrassed to have Joe visit him in London; he rarely visits the forge, and when he does, he belittles Biddy for some faults that are really his. Because Pip aspires to be worthy of Estella, he rejects his boyhood love.
Further, when he learns that Magwitch is his benefactor, rather than Miss Havisham, Pip is repulsed and wants to be rid of the old convict as swiftly as possible.