Even though my profile photo isn't there some people are still good at connecting the dots. If there happens to be someone I know here who happens to follow my posts, or comes across them often, I risk being known who I am in real life. I have a complicated life with several things going that I just dont want to risk a friend figuring it out that its me. And risk of being judged on some things from fellow gag'rs.. Even if it doesn't matter what others think, it does to me.
Sometimes, and its rare, I go anon because if I don't I suddenly get all sorts of messages from guys asking about my sex life and shit. Its like, I told what I did in the opinion because it was relevant to the question, not because I'm a freak that wants every guy on the internet to bother me. I don't do internet dating of any sort. Bit for the most part, I don't do anon. I don't see the point.
Generally people don't want things to be revealed or the situation is too embarassing, as you said. I think I just did it 3 times in my almost 1400 opinions. I don't even remember what was asked, it was right after I registered.
generally 2 reasons 1) if I don't really want people to presume things about me because of the question asked or 2) I want people to answer honestly and straight forward. sometimes knowing the question asker influences answers.