There is also: post something negative/semi-confidential about $work without implicating the work-place or revealing yourself; or ask something you really are supposed to know already without making yourself look like an idiot where your manager/rival can see it.
I like being able to say work is a drag right now because XYZ without actually trashing my work-place or letting hackers know where they can find an exploit or something I describe; I recently described my last work-place as having a cookie exploit which could delete the production DB; if I'd done that under my real name, anyone could read my resume and destroy the site. So "anonymous" posting lets me both vent and be responsible. This is 90% of why I keep myself anonymous which I do behind a username because I like my investment here. I just like it with a side of privacy. :)
That said, I also think catering to switching anonymous on and off is a mistake. I think it's nice that it's possible but it shouldn't be easy.
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