A very good question that really identifies the central theme of this novel - that of a Victorian woman gradually becoming independent - both from the restrictions of society and in her own life, and therefore developing her character until she "comes of age".
Throughout the novel, from her childhood, it is clear that Jane is a character who is fiercely independent and also who is incredibly forthright in what she says. As she comes to know herself through the action in the novel, she demands to be accepted for who she is. She clearly is not accepted by the likes of the Reeds and then by Mr. Brocklehurst, but with Rochester she has found someone who accepts her for the independent and intelligent individual she is. It is highly significant (and feminist critics have had a field day discussing this) that Jane only gets together with Rochester when he is in a position of dependence on her and needs her to look after him.
Arguably, her discovery that she has been left money likewise helps her "come to age" as she is financially independent and able to make her own life choices without having to find employment. It is hard to overstate what this would have meant for Jane - she would need any husband to marry and could choose to be single. It is perhaps ironic that this inheritance comes after all the hard lessons Jane has had to learn.