Just speaking from reddit experience, the Ask Reddit sub there tended to wade into controversial waters. Sometimes people felt the burning urge to answer one of those questions like "Have you ever cheated on your ex" "Have you ever hired a prostitute" or "What have you done at work that should've gotten you fired" as a catharsis, or a need to get it off their chest, or just to brag and in those cases I think the throwaway just felt like an added layer of protection. Truth be it told I'd lived in my reddit account for long enough that if you looked through my post history and were determined and bored enough you probably could've found me other places online or my friends or family could've recognized me if they knew who they were looking for. The most scandalous thing they probably would've learned would have been terribly bland but some people have more exciting lives that they want to share and if those details made it back to them then there could be ugly repercussions.