How is biddy a blessing to the household in great expectations?

Biddy first appears in the book as a helper at the local village school that Pip attends when he is young. Later on, Biddy comes to Pip's house to help care for Mrs. Joe. At the end of the book, Biddy marries Joe, the man who used to be married to Pip's now deceased sister who was always referred to as "Mrs. Joe."