This Quora post lists a few reasons: http://www.quora.com/What-are-the-incentives-to-write-review.... (Some reasons include using it as a food journal, narcissism, reward/punish for good/bad service, and getting invited to Yelp Elite parties.)
This winds up being why I find Yelp reviews to be useless; the poster is trying to tell me about themselves, rather than actually tell me something useful about the restaurant.
"Believe me, I know -- my grandma is a south east indian polar bear, and I know good vegan low-carb empanadas! Also, the service was terrible after I told the waitress she looked fat in that outfit. How rude. Did I mention the parking situation is just horrriiible? 1 star! I know it's always hard to park in the haight/midtown/wherever, but pleeease."
I think the idea is that as you use the site, if you find someone whose tastes seem to match yours and you can relate to the stories/backgrounds that they include on the site, then you can better trust their reviews for places you haven't been, similar to how you might trust a friend's recommendations over that of a random person; it also gives you a guide on how to filter their recommendations.
In this case, the personal information can be helpful to you in that you know you can place less emphasis on the review.
The result seems to be reviews that value the restaurant relative to the effect that visiting and reviewing the restaurant has on the writer's ego.
This seems to result in a dearth of objective reviews about the food (or service), and means that I largely can't identify with any of the restaurant reviewers on Yelp.
A big difference now is on mobile devices. A long review isn't useful on-the-go, where quick judgements are desired.