In addition to the high quality response above, here are some more functions of conversation and dialogue in the novel Pride and Prejudice:
Instead of the unrealistic dramatic monologues of earlier Romantic novels (e.g., Frankenstein), Austen achieves more realism in her characters' short, witty banter and small talk.
Character matches are revealed by their dialogue: Mr. Bennet is a wry wise-cracker, full of sarcasm. Mrs. Bennet is a hysterical drama queen full of overstatement. They are not a good match. Their conversations result in disagreement, but Darcy and Elizabeth's result in accord. We can tell by their like-minded tête-à-têtes that they love each other. So, conversation, rather than parental arrangement or socio-economic status, is the best matchmaker of all.
As the novel is a comedy, conversations are meant to be funny through their use of verbal irony (sarcasm, overstatement, understatement, and litotes). Observe Elizabeth and Jane's back-and-forth from Chapter 4:
"Oh! you are a great deal too apt, you know, to like people in general. You never see a fault in anybody. All the world are good and agreeable in your eyes. I never heard you speak ill of a human being in your life."
"I would not wish to be hasty in censuring anyone; but I always speak what I think."
"I know you do; and it is that which makes the wonder. With your good sense, to be so honestly blind to the follies and nonsense of others! Affectation of candour is common enough— one meets with it everywhere. But to be candid without ostentation or design— to take the good of everybody's character and make it still better, and say nothing of the bad— belongs to you alone. And so you like this man's sisters, too, do you? Their manners are not equal to his."
This dialogue is meta-dialogue (conversation about conversation) and sums up the above points best: conversation is best "candid, without ostentation or design."
Austen is the queen of detailed, elaborate, flowery, lengthy conversations between characters. This is why many people struggle with her novels, and why others love them. A conversation can start on one page and ten pages later still be going. Consider Austen's background and life herself; living in England in a time when women were expected to sit around and do practically nothing all day, what else did they have to do besides talk?
Dialogue helps relay crucial and important information, shape the characters in the novel, introduce conflict, and resolve problems. One major role it plays is in shaping the characters of the story. Austen always has a garrulous and excessively chatty character that says foolish things and is a bore and annoyance to everyone around them (Mr. Collins, as an example). She likes to use conversation to shape those types of characters. Her heroines (like Elizabeth or Jane) are often more limited and wise in their conversations, being the listeners as opposed to the spouters.
Also, consider also how much of the NON-conversation narration in the novels centers and focuses around conversations that were just had. The characters not only have long conversations, but then go home and sit there and analyze every tiny thing that was said in that conversation for potential hidden meaning. So even though they aren't talking constantly, most of the story IS centered round people talking OR analyzing the talking that has occurred. It reminds me of a group of teenage girls talking about a party that a cute boy talked to them at, and they all analyze and interpret, and read meaning into each and every word that poor boy said. That is what the characters do.
So, whether is it actual talking to shape characters, introduce conflict or resolve it, or if it is the characters thinking about what was just said, conversation is the main driving force in Pride and Prejudice. I hope that helped; good luck!