Some legit, sane uses for sock accounts, when you already have a social media presence:
1) OSINT or pen testing
2) Escaping domestic abuse
3) Organizing a labor movement or protest
4) You're 15 years old, and have helicopter parents
Illegitimate uses:
1) Acting 15 as an adult
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I don't call any of 2-4 socks, though. The name sockpuppet implies you're playing the role of a character who's not yourself, not a part of yourself that's not safe to associate with your existing identifier.
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The key there is playing a character (deception). If you're not pretending to be someone you aren't and aren't using it within the same communities, to contact the same people, to promote the same things, etc. then it's just a secondary, anonymous account not a sockpuppet.
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So, for example, maybe someone doesn't feel comfortable contributing to open source projects or publishing other kinds of work with their real identity.
Doing that all under a pseudonym (not a fake identity) isn't deceiving anyone unless there's some kind of problematic overlap.