Not sure why this got downvoted. I am serious. It is easy to seat at your keyboard and demand for people to invest time to write articles/posts etc but why would they? Wouldn't they be better off learning new things, spending time with friends/family, working for money?
He is not demanding. He is pleading/asking politely. "Please". It is totally up to you whether you want to do what he asks for you.
Granted, I agree with you to a point and I don't see any reason you are downvoted/beside the obvious "don't be so negative" point, though I will upvote you, I feel this conversation is important/. At the same time it is extremely easy for you to use these articles/posts even in your CV and in building in your brand. Of course, you might not need that, but there are still a lot of uses including showing them to your future kids or thinking about how your writing changed someones live.
Leave that, there are specific content, which I have no issues being behind a paywall. You might put it behind a paywall/e-book/, show me that your content is useful to me, I will pay you in a heartbeat. Of course this is something that will take even more time.
To your point above spending time with new things or friends/family/working, I don't think that the author wanted you to stop doing them. Again he is asking you politely, do that if you have free time AND you are willing to help some complete strangers online. It is not something everyone would want to do and this is completely okey.
Why does anyone comment on the stories posted on HN? Why do we have leisure activities? I don't think that anyone is commenting on HN in lieu of learning, spending time with family or friends, or pursuing a career. (Though many may be procrastinating at work!) I think the request was a less a demand, and more a polite suggestion for more content of this nature. No one will banish you from HN if you don't participate, but the site and its users might reap the mutual benefit if you do.
Also, I think there is something to be said about the whole "learning as you teach" mentality. Often times I find myself explaining something (which I've done a hundred times) to someone (who has never done it before) and I catch myself saying things that I had not paused to reflect on before; or, I find myself finding even better ways to do things as I explain them to someone else. Those are the sorts of benefits that might be in store for yourself, without any consideration to mutualistic benefits.
"nothing in it for them" is debatable, depending on who you ask. The feeling of pride/fuzzies/etc. felt when doing something on a volunteer basis, the inflence over / trust from someone you've volunteered for, going to heaven, earning good karma, etc. could be counted as "something in it for [the volunteer]".
