The ultimate goal for a young woman in Victorian London and during Jane Austen's time was to come out into society, meet a suitable gentleman to take care of her, provide her a home, and make her "somebody". Taking it from that perspective, that would have been the number one reason Mrs. Bennet needed to "marry them off"- Other than that, the women would be a hassle to society by becoming old maids, with no social position. More than merely marry off her daughters, she wanted to "marry them well", which means basically, to marry them to money and position. We have to keep in mind that her plans continued to backfire because she was a very annoying, obnoxious, loud witch of a woman, and the very people she tried to lure (the upper class) would turn their backs on her middle class self. Mrs. Bennet is also aware of her status as a middle class Victorian. This, although was not at all a poverty status, it did lack the "glitz and glamour" of the gentlemany and ceremonious society of the rich. Also, her upbringing was obviously non ceremonious as her mannerisms fail every canon of admiration and social acceptability- Surely, she wanted her daughters to need for nothing, and "move upwards" in society.
In Austen's Pride and Prejudice (1813), Austen is preoccupied with the phenomenon of social mobility. The family of Elizabeth, or Lizzy, Bennet, Austen’s heroine, is a good case in point. Lizzy’s father (Mr. Bennett) is a gentleman. He lives on the family estate, which provides him with an annual income of about £2,000. Lizzy’s mother (Mrs. Bennet) is from a slightly lower class. Her “people” are professionals and merchants—respectable and decent but not quite on Mr. Bennet’s level. When we look at Lizzy’s parents, we can see subtle examples of social mobility: He has married down, while she has married up.
There’s more to the story than that, for although Mr. Bennet is indeed a gentleman, his position is in no way secure. The family estate can be passed on only to male heirs—and the Bennets have had only daughters, five of them. The business of the novel, as Mrs. Bennet realizes, is to get at least a few of those daughters married off to reasonably wealthy men. Thus, although the novel presents us with some conspicuous examples of upward mobility—one local merchant has recently been knighted, for instance—looming in the background is the awful possibility of downward mobility for the family, and that makes the novel more suspenseful.