I think it's probably because it allows be to write in the most naturalistic way. I like to have my characters speak exactly the way real people speak, complete with ums and pauses right there on the page. I could do that with the narrative too -- and have, on occasion -- but it doesn't really fit. The narrative is about describing what's happening to the reader, and except for certain stylistic choices clarity is usually the best way to go. But the dialogue is about the characters speaking directly to the reader, so all bets are off: if the character is profane, then by all means include swears. If the character has trouble making herself clear, then put in every "but" and "um" that you'd hear if you were really talking to her.
I wrote this the other day, a scene with three friends chatting from the NT:
"What are you reading?" Alex asked Melinda. He'd told Sam the ten or so cases of books he'd brought up with him were only the ones his parents had allowed him to take. Another dozen cases easy had been donated to his local library.See, I think that's pretty funny stuff, if I do say so. But more to the point, I think it's actually how a group of friends would actually be talking to one another: they'd be making jokes, and casually cracking wise.
"Tolkien," she said, bringing a smile of joy to Alex's face and making Sam roll her eyes in despair.
"Please tell me you don't watch his dorky TV shows too," she begged the girl.
"Oh yeah? Like what? My dad just got me the first season of Lost in Space for my birthday."
"Wow, old school," he said, impressed.
"Please kill me now," Sam implored the heavens.
"Pardon her," Alex said. "Sam is wondering if you might also watch some of her shows, like Jailbait Daughter Swap or Pee in a Bucket for Cash or whatever's on Fox tonight."
Melinda was cracking up, and Sam was hard put to maintain a straight face herself. "Oh my God, I cannot believe the frikkin slander I'm hearing. And you're such a loser, because everyone knows Pee in a Bucket is on tomorrow night."
It's certainly what I'd be doing.
Of course, dangers lie therein. My characters tend to run off at the mouth, and if I don't shut them up they're liable to just babble on for page after page, talking about whatever it is I happen to find interesting while I'm writing it. And though I suppose that could be interesting for the reader, it has the effect of leading the reader away from the plot. And while writing dialogue that advances the plot isn't particularly difficult, there have been times where seeing the difference was the problem.
And that's a problem with writing dialogue with such a naturalistic style: irrelevant babbling that wanders off topic. I've sure I've deleted more dialogue than I've written over the years, and while the stuff that ends up on the cutting room floor usually does so for a good reason, it's still sometimes a bummer having to remember that I'm theoretically writing this for someone else, and not my own private amusement.
Kind of like this blog.