I think it must also benefit you, otherwise you wouldn't be doing it, basic human needs are tapped by the quota model (and here on hn too), recognition, self expression, philanthropy etc. Thanks to the subtle game mechanics employed by quora (and hn) these needs are exploited to create a symbiosis, you help them create content that that they can use for their purpose and in return you get those needs met, and create habits around them thanks to the dopamine reward cycle.