Quote 1
"And we mean to treat you all," added Lydia, "but you must lend us the money, for we have just spent ours at the shop out there." Then, showing her purchases—"Look here, I have bought this bonnet. I do not think it is very pretty; but I thought I might as well buy it as not. I shall pull it to pieces as soon as I get home, and see if I can make it up any better." (39.3)
Ugh, Lydia. This is bad news for Wickham (of course, Wickham is bad news for her, too). Pro tip for the wife-hunting nineteenth-century gentlemen: make sure your bride-to-be can manage her money.
Quote 2
"MY DEAR LIZZY,
"I wish you joy. If you love Mr. Darcy half as well as I do my dear Wickham, you must be very happy. It is a great comfort to have you so rich, and when you have nothing else to do, I hope you will think of us. I am sure Wickham would like a place at court very much, and I do not think we shall have quite money enough to live upon without some help. Any place would do, of about three or four hundred a year; but however, do not speak to Mr. Darcy about it, if you had rather not.
"Yours, etc." (61.7)
Here's one reason to care about the family of the person you're marrying: they might constantly be asking for money. But here we see how important marriage is to maintaining your class status. Elizabeth is still a gentlewoman; Lydia, not so much. (If she ever was.)
Quote 3
And when her sisters abused [the bonnet] as ugly, she added, with perfect unconcern, "Oh! but there were two or three much uglier in the shop; and when I have bought some prettier-coloured satin to trim it with fresh, I think it will be very tolerable. Besides, it will not much signify what one wears this summer, after the ——shire have left Meryton, and they are going in a fortnight." (39.3-4)
Truth: we all know someone like this. And, just like now, some (thankfully, a lot more than today) people thought that all girls were ditzy idiots who wasted money on clothes and thought only about boys. Pride and Prejudice was so revolutionary in part because it showed that women could be lots of ways. (Check out "Brain Snacks" for a fun quote about that.)