1) A lot of users on here hold grudges and if you just so happen to answer a question unanonymously or ask a question unanonymously that holds an unpopular view on GaG or that is not so politically correct, you can expect many users to hold it against you
2) I know of a couple of users who only post anon because they don't want to receive any friend requests or messages. They're only on here to answer questions and offer help, nothing further.
3) I guess for extra identity protection. I mean, a lot of people on here post unique situations that aren't likely common...so, it's easy for someone involved in the situation to recognize who would be asking it...but it'll happen whether you're anon or post unanonymously
4) There tend to be quite a few perverts on here, so many girls feel inclined to post anon in the sexuality section
There are some legitimate reasons that people would not want to sign their names to the questions or answers on this site. I have offered advice for a few years on here, good bad or otherwise. The biggest reason I would post anonymously is that I have had people take offence to something I have said and then they followed me around arguing with everything I posted or down arrowing me and arguing with me from then on. I finally had to block 5 or 6 people because of this. I was almost kicked off the site because of the constant harassment from those few people. If I don't want to show my name I wont. Somethings are just so controversial I'm not interested in taking heat because someone does not agree with EVERY thing I say. I mean people are entitled to there own opinion but then again so am I.
I answer anonymously when I'm saying something I think will be unpopular or not believed, and I don't want requests or messages about the question.
I ask anonymously when either I feel the question will be controversial or the situation is personal and I don't even want strangers being able to connect me with it. I have been harassed (and I mean badly) in more than one question and I wouldn't have liked had I not been able to hide behind being anonymous.
Some people don't want to be embarrassed if they ask a stupid, or creepy question. When they give a personal answer, they don't want to reveal who they are. It's like when you watch the news, when the crew asks people on the street (for example) to comment on something that happened, some people want to say their opinion, but don't want their face and name to be shown so nobody finds out who they are.