My reference to her being useless is more in terms of her character - she is described as putting forth no conversation, no effort beyond doting on her children and running a proper household. As for her function in the story, I think she serves a few purposes so far: she makes Sir John a respectable family man, and she provides a reason for Mrs. Jennings to be part of the family so that she can play her role in the story. It will be interesting to see if her role develops at all as the story continues, or if she really is there just to create a circumstance which allows the "more important" characters to exist.