OC stands for "original content" and the feature is a hold-over from how mastodon does things. the idea being that you mark it if you yourself made it and are not reposting it.
no, the people who started mastodon just have a lot of "tumblr" culture. they also have "trigger warning" options, self-censor/blurred pic options, etc. Just a different culture. A lot of kbin users I'm guessing are ex-redditors where we don't really do that sorta stuff lol.