I think the three groups you list match the motivations why someone would systematically (not just a one-off) put in their time to answer large quantities of te

I think the three groups you list match the motivations why someone would systematically (not just a one-off) put in their time to answer large quantities of technical questions without getting paid for it - it's because you really want to evangelize something, or you want to push a dogma you care about, or you're the type of person who gets off on arguing intricate details.