Social proof is to give credibility, reviews are to make decisions. This is for social proof. Also, by definition social proof is positive or else it wouldn't be valuable.

It's an easier way to source all of those testimonials that people have on their website and really cool because it shows that people "like me" value the product. If they do their job right, you could have a testimonial that is more valueable than anything you could have sourced yourself because you are surfacing that diamond in the rough. Especially if you a big brand with thousands, or millions, of people tweeting at you.

Cool prooduct, nice work guys.