Miss Havisham wants to gain revenge on the whole of mankind for being jilted on her wedding day all of those years ago. She doesn't deliberately set out to fool Pip as to the true identity of his benefactor, but he's certainly collateral damage in her one-woman crusade to get back at men for all of the years of suffering and heartache she's endured.
Not unreasonably, Pip draws the conclusion that Miss Havisham is responsible for his good fortune, for his moving up in the world and becoming a gentleman about town. There's no one else around who would fit the bill. Pip would certainly never have dreamed in a million years that it was Abel Magwitch, the escaped convict, who's really his benefactor. Although Miss Havisham may not have planted the idea in Pip's head, she doesn't exactly go out of her way to disabuse the young man of his erroneous conclusions. So long as Pip thinks that Miss Havisham is his benefactor, he'll be more likely to pursue Estella, and the more he pursues Estella, the deeper he'll be suckered into hers—and Miss Havisham's—elaborate trap.