We have found value in reading through job candidates' StackOverflow answers and comments — not for knowledge, but for personality. We can learn a lot about whether someone has the right temperament for our team from their answers.
Reputation doesn't measure much other than time spent, and there are few questions on SO relevant to the work we do. But someone's social nature will show through in everything they post regardless of content.
Are they polite, rude, articulate, or indecipherable? Do they treat others with respect? When someone disagrees with them, do they respond collaboratively, aggressively, or sarcastically? Do they acknowledge when they're on the wrong side of a debate, or fall back to arguing semantics and ad hominem? Are they pragmatic or pedantic? Do they show signs of immaturity or poor anger management?
An employee with the wrong personality fit is more toxic to the team than a simply inexperienced one, and it's hard to get a real sense of that from just an interview and some references. Looking at someone's postings here, in their natural habitat as it were, is an opportunity to see what the "real them" is like.