Why is it important that Jane inherits wealth before marrying Rochester?

There are several important reasons Jane inherits wealth prior to marrying Rochester in the novel "Jane Eyre." First and foremost, as the sole heir, she is able to disperse some of the wealth and provide for her less well-off cousins, and give them independence.
Additionally, by receiving this money, she becomes independent—she has her own wealth and can, legitimately, choose to do whatever she wants. Prior to this, she was a working woman, which was very rare in those days, and she wanted to have control over her own life. This allows her to choose to marry Rochester for love alone, instead of marrying him and becoming dependent on him.
Finally, Rochseter has lost everything he has by the end of the novel, with his estate burned to the ground and his vision destroyed. Jane actually acts as the provider and savior in the story by marrying him when she will be providing wealth—although she knew he loved her because, when he was wealthy and she had nothing he proposed marriage to her.

During almost the entire period covered in the novel, Jane is a pauper. She has her intelligence, integrity, and education, but not money nor beauty. Jane has had to work for a living, which only poor women did in that time. Yet because she has retained her integrity, she has declined marriage to St. John, whom she did not truly love. Because her work as a tutor and governess would not earn her a living wage, the only viable way for the author to account for Jane getting enough money to live on is to have her inherit it. Whether she married Rochester or not, having enough money was essential for her well being.
Once Jane knows that she is independent, she can make her own choices. Rochester, who was both handsome and rich, could not understand Jane’s earlier reservations about being with him. After his injury, he turns the other direction and sees himself as unworthy of her: too much of his self-esteem had been bound up in his good looks and able-bodiedness. Jane, however, knows that looks do not matter. What is important to her is that she is no longer dependent on any other person, which frees her to choose to join her life with his.

Jane inherits 20,000 pounds from her uncle in Madeira, which she insists on dividing four ways with her cousins Mary, Diana, and St. John Rivers. Five thousand pounds, which would bring her 250 pounds a year in interest to live on, leaves her independent.
Jane can marry Rochester as an equal partner now that she has enough money to live on. When she was about to marry him earlier, she had no money and was dependent on his largesse, which made her uncomfortable. She didn't want all the wedding finery and didn't want to be put into an inferior position. It was also very difficult for her when she fled because she had no money and could have died, had not Diana and Mary rescued her.
Now, with her own money, and with Mr. Rochester blind and with one arm crippled, Jane, though a woman, can have a relationship based on mutual interdependence rather than the hierarchy of male over female which was common at that time.