Is there a Captcha FAQ which explains why it's used when you're logged in? I understand, in general, they are used to stop comment spam bots but when you're logged in, it seems there are other considerations? Like slowing down users?
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The only purpose is to stop bots; there's normal rate limiting used to stop you from manually doing things too fast. For example, if you vote on comments too fast you'll get:
You won't get a captcha for that; it just tracks when you last submitted a vote and stops you completely from submitting another one too fast